It's important to remember that although Roosevelt was effectively paralyzed from the waist down, he could stand with the aid of leg braces and even walk short distances with the aid of a cane and/or people helping him balance. He could also stand at a podium and deliver speeches by using braces and holding the podium. Public appearances with roosevelt were carefully orchestrated to conceal his handicap. He was often photographed standing, but with a cane, or leaning on someone sort support. this shows his cane and some pictures of public appearances of him.
It's also relevant that even when Roosevelt was first elected president in 1932, he was 50. Not old exactly, but old enough that having a cane wouldn't have been terribly out of the ordinary. There are many reasons a 50 year old might walk with a cane or limp around that don't amount to total disability that would somehow make him unfit to govern. (hell, I've been told by a doctor that I'll probably be a candidate for a knee replacement by my 50's).
However, more recent historical work has attacked some of the conventional wisdom. The myth of FDR's secret disability
An NBC Nightly News report on the discovery stated that there was “a gentlemen’s agreement” between FDR and the press corps to hide the extent of his disability, and the Associated Press wrote that it was “virtually a state secret.” That has long been the conventional wisdom, repeated in countless books and articles. But it is inaccurate. In fact, the press sometimes described his condition in great detail.
A 1932 New York Times Magazine profile of the then-Governor of New York described how, at his Hyde Park home, he “wheels around in his chair.” A TIME article from February 1, 1932 said that swimming and exercise “have made it possible for the Governor to walk 100 ft or so with braces and canes. When standing at crowded public functions, he still clings precautiously to a friend’s arm.” Before his inauguration as President in 1933, TIME noted, “Because of the President-elect’s lameness, short ramps will replace steps at the side door of the executive offices leading to the White House.” And a TIME article from December 17, 1934, described a scene in which “bodyguard Gus Gennerich helped the President into his wheel chair, rolled him the length of the West colonnade to the new White House offices.” Another profile, this one in the New Yorker in 1934, stated that “he is almost always pushed to the west end of the White House in a small wheelchair.” Seven years later, on January 20, 1941, LIFE magazine noted that “by 11 o’clock he is up, dressed and on his way in a wheel chair down the long passageway to his office…” (Not that it slowed him down in any way. The headline for the article was “Roosevelt: From Breakfast in Bed to Wisecracks at Movies, President Retains His Bounce after Eight Years.”)
As for incriminating images, it took far more than a “gentlemen’s agreement” for the FDR administration to discourage photos and newsreel film of the president in his wheelchair. Rather, the Secret Service used force. As Editor & Publisher reported in 1936, if agents saw a photographer taking a picture of Roosevelt, say, getting out of his car, they would seize the camera and tear out the film. “By what right they do this I don’t know,” the correspondent wrote, “but I have never seen the right questioned.” A 1946 survey of the White House photography corps confirmed this, finding that anyone the Secret Service caught taking banned photographs “had their cameras emptied, their films exposed to sunlight, or their plates smashed.”
This shows, that there was certainly some public knowledge of his disability, but it's unclear realyl how widespread this was.
As to whether his disability made it into foreign propaganda, it doesn't seem to have been the focus of much, if any. this article has a fairly significant discussion of Roosevelt's perception in Germany during the war as well as a nazi prpaganda poster showing roosevelt standing to put an uncle sam tophat on the globe - it cites a 1941 book of collected german cartoons, and says of Roosevelt "a puppet of the Jews, as a warmonger, or as a failure with his New Deal, but his disability is hardly evident."
For example, the book's front cover features Roosevelt admiring his mirror reflection as a Jew. Other cartoons show Roosevelt and Churchill as puppets of the Jews, a blind Roosevelt being led by a Jew (this from a U.S. pro-Nazi newspaper), and Roosevelt being ridden, and thus controlled, by Jews in a "Jewish kindergarten." Another group of cartoons combines antisemitic imagery with attacks on America's supposed belligerence and desire for world domination. The back cover shows a standing Roosevelt putting Uncle Sam's hat over the entire globe (Fig. 4), and another cartoon transforms his famous toothy grin into a mouthful of bombs. One particularly loathsome cartoon depicts Roosevelt holding a barely visible cane while supervising Jews who are torturing the American people and preparing for war. Finally, the magazine of satire called Kladderadatsch published the only cartoon I know which shows Roosevelt with two canes, headed down the "path of Wilson" into war (Fig. 5). It appears that the image of Roosevelt's disability in the Nazi press did not change in any significant way as the war progressed. Later, in newspapers such as Das Reich, there were relatively few cartoons depicting Roosevelt, since the U.S. was usually portrayed as an ugly Uncle Sam, along with Britain's John Bull and France's Marianne.
There is, however, some evidence of knowledge as described in the article.
Undoubtedly there was a certain amount of knowledge abroad, including in Nazi Germany, about Roosevelt's physical condition, although it is not so clear whether the extent of his limitations was recognized. One graphic statement was made by Mussolini on the occasion of a speech by Roosevelt that Italy's fascist leader especially disliked: "Never in the course of history has a nation been guided by a paralytic. There have been bald kings, fat kings, handsome and even stupid kings, but never kings who, in order to go to the bathroom and the dinner table, had to be supported by other men."[7] In Germany, comments on Roosevelt's disability seemed to focus mainly on the supposed connection between physical illness and mental or emotional condition. The Nazi editor of the above-mentioned book of political cartoons, for example, referred to FDR's bout with polio by saying that "After a few years his affliction improved so that he was able to return to the political arena in 1928, but the struggle with his illness not only increased his high self-esteem, but also increased his desire to be active in a very dangerous way."[8] And he went on to say that it was probably possible to understand Roosevelt's mentality only by realizing that he was "a physically broken person who is constantly venting his hysteria
We can conclude, I think, that there was some knowledge of his disability. The leaders of foreign countries probably knew Roosevelt was "lame" so to speak, but maybe not the true extent of his paralysis. And for whatever reason, they thought it far more advantageous to claim that being physically "lame" made him crazy, than focus on him being disabled as a propaganda point in and of itself.