Hey Hans,
Info for the game is packed into ".res" files. These can be accessed by:
1) opening up salam-res.jar in a program like 7-zip or WinRar (?) (I use 7-zip, so WinRar is a guess)
2) extracting the contents into some appropriate location
Inside this file you should have various folders, but the gfx (for "graphics") is the one that will be most of interest to us. gfx has various folders (kritters (creatures), terobjs (terrain objects), tiles (terrain tiles), etc.), and each one of these has various .res files (eg. bale.res under gfx/terobjs, which is the information package for the bale objects seen around Boston).
The info from these files needs to be decoded in order to be accessed. The best, most up to date tool for this is boshaw's Salem Layer Util (he just updated it in the last couple of days), which can be found here: http://odditown.taggedup.com/boshaw/Lay ... salem.html
You will need to do some googling to get setup.
Once these files are decoded or unpacked, you'll be able to access the images associated with each object too. You can use various tools (photoshop, etc.) to replace the images with a custom image, etc. Once you are done changing the images, you will need to repack them. boshaw's Layer Util program can do this too (read the homepage to see how).
Once you are done this, you will have to get the game to use the files you want. The easiest way I know of is that if you are using the custom gfx mods from this forum, just drag and drop the .res files you have in the relevant locations under custom_res, as the game is already set up to use that folder.
The tricky part here is getting lisp etc. setup, and how easy this is for you will depend on your experience/google fu/patience. Feel free to PM me or Chiprel (who guided me and was very helpful) if you get stuck and need help. Good luck!