the best textures for you to be playing with are the battle textures for locations.
check on the reconstruction sheets battle.lgp section to find the textures of the battle locations (floors mostly)
reconstruction sheet linkthe battle ones start @ oh** textures. ones in the first area would prolly be best since it would help w/ the current team avalanche bombing mission project
link. using the pictures on the sheet as a referance start a new picture at 8x the textures size and add the picture as a background layer (scale it up accordingly) and then create a new layer over it (and use the transparency of the layer to see the background as a guide for your new texture) when your all done you can play w/ it in game by saving it as a .jpg(you might want to scale it down to a smaller size based on your play res, but keep the huge one for TA to use they will scale a few versions for the low and high res patches) and converting it w/ imgtotex save it as the file your replacing. then extract your lgp file back up the orringal texture (just incase) and replace it w/ you new one then pack the file back up and try ingame (DO NOT DELETE YOUR .PSD FILE).
you can extract your lgp files using aali's lgp tools (its a cli program) grab my installer
link install the version for your so (prolly windows). and then run a command prompt. and use
C:\unlgp.exe <sourceLGParchivepath>
to extract ( i recommend copying your lgp file to a new folder since it will extract the files to where the lgp is located.
to repack the archive its bascily the same thing
C:\lgp.exe <sourcefolder> <outputfile>
please remember to remove the orringal .lgp file from the folder before you attempt to recompress the lgp. also note if you install my version you can browse to where the files are located using cd and still use lgp and unlgp (since they are installed in /system32).
hope that helps.