Although there are certain rules that you have to follow before submitting pictures otherwise they'll be rejected :/. Things such as ...
*. Image resolution.
*. Aspect ratio.
*. The container format (PNG, JPG, etc) and if you deal with vector graphics, then make sure to only use the SVG format.
*. Avoid excessive use of the Ubuntu logo are just a few to mention.
You can use proprietary applications such as Photoshop and Illustrator but the wiki page (link below) has an interesting quote...
"Work generated with proprietary applications like Photoshop and Illustrator is acceptable, but files in their formats are useless for many of us..."They also emphasize on not using text since they attract the attention from "whole image" to themselves (oh those self centred *$*& :/) + they may not look good after zooming, etc as well.
If you're interested, then you can read more from this Wiki page which includes a giant list of things you can't do :/. Enjoy!.
Note to all the artists out there - Guys if you feel a bit offended by the way I wrote this post, then I'm sorry, no such thing was intended, it's just my attempted humour :).