If you're a dictator, l'état c'est moi is the real deal. So if the nation requires glorification, you must humbly submit to at least a small dose of same.
In the age of photography, portrait painting seems to have taken a back seat to the lens and darkroom where dictators are concerned. Nevertheless, paintings were produced for many of the leading dictators of the era 1920-50.
My guess is that the Soviet Union's Stalin was depicted in paint the most. These paintings weren't not necessarily portraits; a good many showed him with workers, children, et cetera gathered around him or in other genre settings.
Stalin and China's Mao Tse-Tung had their faces on huge banners, but that's something different from a formal portrait; our Mao example is in fact a poster. Adolf Hitler, despite of or perhaps because of his background as a painter (of architecture, mostly) tended to favor photographers.
Below are examples of dictator portraits.
Joseph Stalin by Victor Oreshnikov - 1948
Adolf Hitler by Jacobs
Mao Tse-Tung poster
Jozef Pilsudski by Wojciech Kossak - 1928
Benito Mussolini by Gerardo Dottori - 1933