That Napoleon was eventually defeated was the result -
in part - of a painting that depicted a giant, rather
inattentive, absent-minded giant eating a human corpse
like a candy bar.
If you are referring to Goya's "Saturn Devouring His Son" then I don't think it was even painted before Napoleon's defeat. In any case, suggesting Napoleon's defeat was "the result" of a painting is hyperbole to the point of absurdity. There are examples of art having significant impact in the world but this is not one.
If you are interested in art and Napoleon (especially satirical prints and their effect on the politics and war of that time) then the British Museum has a temporary exhibition on "Bonaparte and the British" showing many cartoons from the period.
