Oops, a bit more explanation:

The actual "painted" file is in a format called TARGA. The file most often comes out of wings named diffuse.tga. The .tga bit at the end lets us know it is a TARGA picture. We need a JPEG or diffuse.jpg.

If you open this file with a painting program (which you'll need to do to paint the texture), you will need a program like GIMP (under redhat linux it is often under graphics programs in the start menu?)

When you've painted the texture, choose "save as..." instead of "save..." in the options and choose what type of file, select JPEG. Or just save your file as diffuse.jpg and GIMP will often detect the ".jpg" part and save as a JPEG anyway.

Then use, umm Notepad? Wordpad? OpenOffice? Word? or Kword? (what do you write with?) to open the "plane.mtl" file. Change the last line from "map_Kd diffuse.tga" to read "map_Kd diffuse.jpg".

Hope this helps.