lol nah, look in your steam folder, in rescources somewhere, open all the 600x800 ones up seperatly, set them on one page line them up then put yours over the top the hghlight the sections of the origional, and whilst you have the origional peices selected, grab your background, copy the part of your back grounds selected make a new doc and paste it there, save it as a .tga and rename it the correct file name.
sorry about that i said it all in a rush, you want a better explnation get me on msn, ive done one of these b4 its a tedious crappy little thing to do