All icons exist internally in a few DDS format images. You can edit these in any image editor that can load and save DDS format. Paint.Net is a free one that supports dds format natively. Gimp and Photoshop are more powerful, and there are free plugins to add DDS support.
The skill images are in one file: skills.dds
The numbers you mentioned correspond to the many 75x75 pixel slots in the image file. (Called an atlas)
You can make your own atlas and add it to the definition of a new skill like so:
(By way of example.)
Code: Select all
iconAtlas = "mod_assets/textures/custom_skills.dds"
For an atlas, you can either create the texture yourself, and insert your custom icons (side by side; each 75 pixels square), or you can use the Grimrock Atlas Toolkit