[.041 Safety Wire]

You need to use photo-editing software such as photoshop or Gimp to open the graphics files. They will most likely be TGA files so when you finish modifying them they should be saved as a TGA file and returned to the folder where the game has them stored unless you are trying to develop new buildings from the ingame models in which case you will need to modify some of the ini files in order to have them as new objects. Hope this helps. There are quite a few formats that games use for their graphic representations these include TGA, BMP, JPEG, DDS to name a few also be aware that some graphics files have Alpha layers associated with them. It would be good to have a manual for you photo editing software plus another on game graphics.