.jpg image format is not meant for pre-1990's computers nor anything with less than a Meg of RAM.
Also, jpg is not well suited to very small icon images which could reconstruct to something which fits within the memory of an arduino.
Consider looking instead at black and white icons or find a project more suited to a small memory responsive microcontroller with plenty of input and output lines.
A string is a really poor inflated representation of the compacted binary of a .jpg
Don't do that.
If you want to learn about .jpg and things like that, try looking up "octave" programming language, which is a big application which could be installed in linux ( and Windows7 the last time I looked ). In octave, type
help imread
and
PPP=imread('imagefile.jpg');
imagesc(PPP);