Crab a unpbo tool somewheres and check one of the BIS SP missions...
There's something like pic.paa, open it with TexView and 'convert' the img to *.tga by saving the opened pic.paa as pic.tga... Then open it in yer Photoshop and u will see how BIS has done it..
Then u can just copy/paste your own image on top of the BIS image, do the necessary scale and flatten thingys and save as *.tga.....
Then open that .tga in the TexView, hit generate from the mipmap menu and save the .tga now as .paa...
Should work... No guarantees, I have a bad flu... ;D