In GIMP, make a 512x512 square and color. Then make a material file. Copy a material file from your material folder, and paste it in your my_objects folder. Rename it, and open it in notepad. Erase everything in it, and put:
material MATERIAL NAME HERE
{
technique
{
pass
{
cull_hardware none
cull_software none
texture_unit
{
texture TEXTURE HERE.png/.jpg
}
}
}
}
So, once you have that in, change "MATERIAL NAME HERE" to whatever you want the material to be called. Like (NameOfObject)Mat. Then replace "TEXTURE HERE.png/.jpg," with your textures name and the type of file it is, .png or .jpg
Then load your mesh in game, and insert whatever you called the material (the "Mat" thing) in the blank below where you load the mesh in.