Here's what to do:
1) Take a picture of what you want for the image
2) Create a 256x256, uncompressed TGA from it
3) Place this in your textures directory
4) Reference it in the XML file; do not include the extension, just the filename.
Age of Mythology automatically creates BTX and BTD files from your TGA, after which you can actually delete the TGA.
A warning, however: custom map image files are not transferred in multiplayer.