For JPEG info goto ijg.org and/or do the usuall internet/google search.
Warning: it is not simple.
Perhaps consider a different picture format.

For displaying VGA (color), you won't be able to do it directly by microcontroller, you need a VGA controller chip.
Have a look at Fujitsu's Jasmine and similar products thereof.
Not simple, either. Nor cheap.

I know nothing on displaying on mobile phone displays, sorry.

Jan Waclawek

PS. I have seen a JPEG decoder written in '51 asm for pictures from a particular digital camera, but lost the link...