Make sure that the fonts that you use are compatible with GPL3+. To be on the safe side, use fonts available in Debian main, otherwise we will ask you to prove their compatibility with GPL3+.
As the logo will be used in quite different places, we encourage you to submit complementary materials. These are not strict requirements but will help us evaluate your proposal better. Besides, if your logo is chosen, we will need them anyway before we are able to put it to use publicly.
A splash screen for boot. Must be 640x480px with a black background, 8-bit/RGBA. It has to be distinguishable by the user as Tails, but discrete enough not to attract too much attention from other people around.