Usually, the best person to speak to about final deliverables is the contest holder. Often, they have specific requirements, which they should have made clear in the brief. If, for whatever reason, they didn’t, you should always feel free to ask them in the contest discussion, or by sending them a private message.

For website designs, Twitter backgrounds and other bitmap images, you should provide a layered RGB file in Photoshop PSD or Fireworks PNG format. It’s a good idea to also provide a flat JPEG version, as this demonstrates how the layered file should appear.

If you have also designed a logo or some other vector image for print, then providing a CMYK version of the file in either EPS, AI, CDR, PDF, or SVG format is your best bet. If the brief called for one-color pantone, or anything specific, you should also bundle that in.

Any commercial fonts included in your design should be rasterized, outlined, or acquired legally by the contest holder.