.rul files are what you're looking for.
you'll want to make a mod, the ruleset for your mod would be something like:
extraSprites:
- type: GEOBORD.SCR
width: 320
height: 200
singleImage: true
files:
0: Path/To/Your/File.png
if you're daunted by the whole process, try downloading a simple mod that does something similar to see how it does it.