Hey, it would be good to etablish some submission guidelines in order to keep the usefulness of the single entries high. Let me know about any suggestions.

Basic 2D:

Format for 2D submissions should be PNG with alpha channel, JPG is good too for high resolution graphics without transparency.

PNG is a very common free format which provides very good lossless compression. Big pictures can get quite a file size for it, so for backgrounds the more effective but not lossless format JPG might be better suited.

If layers were used and are usable the source files (.psd for Photoshop, .xcf for GIMP, .kra for Krita, .ora for MyPaint) should be included in addition to the PNG/JPG files.

Concept Art:

While not required a model sheet would make it a lot more useful for 3D artists. Same goes for detail drawings for equipment or other smaller parts of the concept. The more details are explained the better. (Example of a model sheet: http://www.davidrevoy.com/data/images/blog/2011/11/littlebotbunny_davidr... )

3D:

3D graphics are usually very specific to the application they were created in and for and export and imports often lack features, so the source files always should be included if possible.

To be complete and useful to the highest degree the characters should be rigged and should include a few basic animations. Most used animations are walking, dying and attacking. Also very useful animations are jumping, staggering, activating and blocking.

Audio:

While MP3 isn't a bad format it is patent protected and using it commercially requires paying fees. OGG provides similar quality at similar size without being protected or requiring fees. The developer can recompress the MP3 to a OGG, however quality is lost this way. So if it is possible to provide a OGG please do so.

Even more useful would be FLAC as it is lossless, it can be recompressed into a lossy format at any time and at any quality. Depending on what it is used for and what quality is wanted.