Propaganda is a funny business. As you so rightly point out, what is great and effective at mobilizing support in one situation looks completely inappropriate in another. These types of things are cultural secretions entirely dependent on time, place, and especially its audience.

Propaganda of any type - both white and black - depends on knowing its audience. "White" propaganda is something like an Army newspaper or a flattering documentary. It is designed to make a side look better than their enemies, or promote positive understandings and behaviour. Far and away this is the most common type of propaganda - even if you look quantitatively at posters produced for the Allies during World War One and Two, they are mostly aimed at encouraging workplace participation, getting people to buy victory bonds, or enlist.

((For America in World War Two, this makes especial sense. The government officially had no organ of propaganda until the Office of War Information in 1942, due to fears over how it would be perceived by the public. The OWI was formed not only from official channels, but also from private industry; it is considered the forerunner to modern advertising.))

"Black" propaganda is the negative type we all think of - the type that makes the enemy look bad. There are actually a few different groups even inside "black" propaganda. There is the type that makes your enemy look like a caricature, promoting ridicule. There is the kind that makes the enemy look effectively monstrous.

This second type - a tiny, tiny fraction of the total propaganda produced - is the type that tends to disproportionately attract the attention.

So, we need to ask ourselves. What is a "monster"? Why do people find them so memorable, and so worrying? The answer, at least according to Judith Halberstam, is that a monster can't be one scary thing, or even a couple of scary things, but instead must be made up of a kind of 'gestalt of fears'. A monster is a build-up of all the things people are worried about, or that concern them, or that secretly frighten them. It is a place where this conceptual fear is personified in flesh.

In a book called War Without Mercy, John Dower traces how Americans conceptualized the Japanese throughout the Second World War. From the start, the dominant iconography of the Japanese was as a jungle Ape. Monkeys and other racist imagery abounded, playing on white American fears of the "yellow peril", speaking to pre-existing concerns about race, and about how threatened civilization was in that part of the Pacific. These fears were enormously pervasive throughout American society, and so the imagery played very well. They play less well now as concerns about burgeoning Japanese power, and concerns about the Japanese in racial terms, don't really exist.

It is a big question you've asked, and I have tried to be clear, but inevitably I have had to shorten what would be a couple of lectures into something which might not answer all you wanted. Feel free to ask any follow up questions you'd like!