Themes will be picked from other 20in20 community or theme-providing challenges.
A few examples (shamelessly snagged from whedon20in20)
Smile Time -- show your chosen character(s) at their happiest. The cheesier the grin the better.
Two by Two -- favorite pairing.
Briar Rose -- mostly pink/rose colored icon.
Categories can be things like 5 icons of your favorite episode, or your five favorite episodes etc.
And Artist's Choice...well, the name says it all.
If you have any suggestions or questions, don't hesitate to ask.