What platform would you run Quetoo on if it was on Steam?:

Textures and Texture Sources

Quetoo supports all popular image formats for world textures: TGA, PNG, JPG, and even Quake 2's WAL format. All of the textures distributed with Quetoo may be reused in other levels, provided you pass along whatever documentation comes with them. Please avoid creating levels with textures that may not be freely distributed.

Here are some sites which provide excellent quality, legally distributable textures: