"It is an irony of fate that I myself have been the recipient of excessive admiration and reverence from my fellow-beings, through no fault or merit of my own" Albert Einstein, 1954

As my work touches the popularisation of certain scientists from a historic point of view, I can give you at least a partial answer. I will only talk about Einstein, hence, but I assume some of my points are applicable to the other celebreties you mentioned. The unsatisfying part is, though, that there seem to be no general rules, why certain persons become famous (and some of them even "inspirational figures"). While a lot of it is simply chance, some points can be identified, though, that facilitate this process.

First of all, for a scientist, it certainly helps to develop/discover something spectacular. This is obviously the case for Einstein, whose theory of realtivity revolutionized the field of physics. Still, this is by no means a gurantee for imediate stardom. Consider for example Antoine Lavoisier. His experiments in the 18th century showed that the so called Phlogiston theory, which was the predominant theory of why and how things burn, was incorrect. He is therefore, among other things, considered (one of) the father(s) of modern chemistry. Have you ever heard of him? Probably not. Most people haven't. One of the reasons is certainly, that Einstein lived in a time where scientists found a significant media coverage -- Lavosier didn't. Just consider this cover of Times Magazine from 1946. But this was printed at a point, when he was already very famous. But overall, the media played an important role in Einstein's transition to a world wide known figure. (The following is basically an inadequate summarisation of an article by David E. Rowe.)

Already in 1919 a very important experiment had taken place. During a solar eclipse Einstein's theory that the gravitational field of the sun bends light was proven and found a huge international echo. Look at the headlines in the New York Times from November 10th 1919: "Men of Science More or Less Agog Over Results of Eclipse Observations", "EINSTEIN THEORY TRIUMPHS" and also assures us, we don't need to worry, because the stars are located at different points than they seem to be (or were calculated).

Another factor for his huge popularity was his popular account of relativity, first published in 1917. Let's face it, relativity is a difficult subject, and one of the premises for widespread fame is something to be known for. In "Über die allgemeine Relativitätstheorie" Einstein explained his ideas in terms that could be understood by layman. He introduced famous thought experiments, like observers in- and outside of traincars, that related to everyday life. The book was a huge success and went through 14 editions until 1922 alone. I have to admitt that I have no idea if this booklet actually features the iconic "E=mc²" (haven't read it in a while, but I'm happy to check), but the fact, that his theory could (seemingly) be wrapped up in a single formula that can be uttered and remembered by everybody, surely helped as well.

I'll only mention the following points briefly, because, boy, this is getting really long :-) and my argument will become clear anyway (Whoever wants more details should refer to the article by David E. Rowe mentioned above):

• During the Weimar Republic a huge number of books were published about relativity. [cf. the article by Hubert Goenner: The Reception of the Theory of Relativity in Germany as Reflected by Books Published between 1908-1945. here, starting on page 15]

• There was also a somewhat significant Anti-relativistic movement at the time, which lead to even more publicity.

• In 1922 Hanns Walter Kornblum made a movie about relativity, aimed at broad audiences. It's program gives (at least a) hint of why relativity was so popular:

"What accounts for the public fascination with Einstein's theory? He has shaken our customary perception and made us feel insecure on this spot of earth that seemed so firmly anchored. This insecurity explains the longing to understand these ideas that have overturned the world". (This view was of course contested by others, but which claim isn't?)

• Einstein gave a famous radio speech at the German Radio Exhibition on August 22nd, 1930 (You can buy a CD called "Verehrte An- und Abwesende" of it, there is an English edition, which of course is senseless in regard to this specific speech, which is in German (it's the original broadcast, duh!).). He gave more radio interviews/speeches, especially after he emigrated to the US. (Some of which are on the CD aswell and in English, so if you want to hear Einstein talk, give it a shot.)

• In later years, Einstein became famous for his humanitary, political, religious ect. views (his scientific phase was basically over in 1919). His already established status of fame put him in a positon, in which he could make his voice heard much more easily. Some of his standings were published in the book "Mein Weltbild" (My Worldview) which was, naturally, a huge success. It was an anthology composed by Einstein's son in law, Rudolf Kayser, who was in dire financial struggle after having to emigrate after the Nazi's rise to power. Kayser had already published a biography on Einstein before, which was nowhere as successfull, though.

(The following is my own oppinion:)

All in all, Einstein's rise to fame was a very idiosyncratic one -- which is probably true for everyone famous. Many factors played a role (some of which surely apply to other celebreties, but neither of them is a gurantee for imediate stardom): A revolutionary theory that seemed to shake the fundaments of science, a sensational proof of said theory, popular accounts that were understandable by layman, the wide media coverage. Moreover, Einstein lived in a time where photographs were frequently published in newspapers (unlike before). And he was photogenic (not only, but definitely due to his hairstyle). As a result, many pictures of him were published, some of them, like the famous tongue-picture, became icons. This picture also shows another side of him -- he was a funny person. His books are full of witty remarks and observations. That makes him likeable. His status a genius also established a certain wisdom in the eyes of the public. Those two factors make it easy to pick out quotations and slam them on posters. Preferably with a funny picture of him.

Concluding, let me add a short note on those quotations. As they are taken out of context, they often tend to be misinterpreted. A good example is “Do not worry about your difficulties in Mathematics. I can assure you mine are still greater.” which is often presented with the claim "Einstein was bad at mathemtics" to ease struggling highschool students. But Einstein was never bad at maths. In fact, his grades were excellent. He struggled with the mathematical formulation of the theory of relativity, and believe me, that's a whole different kind of difficult :-)

(In passing, I can also recommend "Einstein on the beach", not the opera (which is awesome, too), but the anthology edited by Michael Hagner. It's in German mostly, though, but it has lovely articles on different aspects of Einstein's fame, such as the portrayal of his hair.)

Perhaps some Art Historian here can connect the dots from religious images to propaganda posters to the rise of photography in the first half of 20th century to Pop-Art of the 1960s to band posters and cultural icon posters.