This should probably go in Artifice & Arcana.
You should use
http://forum.salemthegame.com/viewtopic.php?f=11&t=5417 (boshaw's layer util from HnH, also available for salem) to unpack .res or .cached files (if you're using a custom client, these are in ~/.salem or in ~/Salem/bin/salem-res.jar). Change the image content, and repack at your leisure.
If you want to overwrite existing resources (from ~/.salem or ~/Salem/bin) when using a custom client, you should place them in the correct tree under ~/Salem/custom_res. So for example ~/Salem/custom_res/gfx/terobjs/... I don't think it's currently possible for vanilla.