For the HTC Phones, most files are in form of .png's. I think the Optimus line (maybe even all LG) of phones have another format. Though I'm not sure which one it is. Most graphics are located in System/framework-res, and can be extracted with .7zip on any PC or Betterzip(?) on Mac.
In most cases you can edited graphics in this location without resigning. Stay away from the ".9.png" files as these require more editing than replacing the image. Hope this helps.