ok so you've extracted .sfs files- that's a good start
there are alot of 109 versions around so it might be slightly different depending on what version you have- is this an extract of 'vanilla' files or from a 'mod pack'?
assuming mod pack:-
for versions that are based on Freddy's work the pic is found in
Bf109P2PanelDMG.tga. this is a file that has a pic already in- just paste your pic over the top keeping it within the outline of the orig pic. Save it keeping the alpha channel and place it in a suitable folder at the correct 'depth' in your MODS (or equivalent) folder.
You will probably have to change the 'tga' file to a viewable format (imf.viewer or similar) before editing in photoshop/gimp photo editor program.
it is easier than it sounds- honestly