All "textures" in Quake 3 are seen as shaders. Loading a simple texture means it gets loaded as the most basic shader of all:
textures/(mapname)/(texture)
{
qer_shaderimage textures/(mapname)/(texture).(extension)
{
map textures/(mapname)/(texture).(extension)
rgbGen identity
}
{
map $lightmap
blendFunc filter
}
}
When you select a texture(with no shader) in GtkRadiant, the extension is cut off, because it is a shader of its own.
Add (mapname) to the end of shaderlist.txt, found in scripts/ so you can see your own textures and shaders.