Personally I would forget GIMP and I would advise to use Photoshop. The primary rule is, check the resolution standard for each book layouts (5,5x8,5, 6x9, etc, etc...), plus include the bleeding if you want to print it. Also use large DPI, such as 300 or 600 if you want a better quality (300 is used to be enough, but sometimes higher DPI is better.). And one of the most important thing, the fonts. You may buy or license fonts, you may use free fonts for your commercial project (Some of the free fonts are for non-commercial projects only. Always check their readme.txt) or last option, you may create your own fonts with font creator. Same goes for pictures as you can't use most of the pictures what you can find on the net. In this case, ask permission to use AND modify it (This last is important and some artist is not going to let you to modify his / her art.). Other option, license the picture (Again, check the license that you have the right to modify it) or ask an artist to create one.
Thinking of asking someone on Renderosity.com for permission to use their pic.
And one last thing... especially in this case when you want to use someone's property. Always match your ideas with the artist. Artists are used to be very sensitive when a non-artist is trying to manipulate his / her art, even if (s)he is giving you the permission to use it (Which is used to be very rare, especially if you want this presumably for free.).
