the idea is to make extensions for different submissions with a special theme or style.

for example:

http://opengameart.org/content/classical-ruin-tiles

http://opengameart.org/content/lpc-skorpios-scifi-sprite-pack (more theme-based)

http://opengameart.org/content/minimal-industrial-tiles (more style-based)

http://opengameart.org/content/desert-tileset

you should make things that fit with that style and/or theme