Working With .IMAGESET Files
The only icons the game will know to use are those it reads from the .IMAGESET files in the game and in mods. All IMAGESET files do is hold X and Y coordinate info of where to pick-up a single icon on an ICON SHEET that contains many icons. It just neatens things up for using icons.
The important bits of info inside an .IMAGESET file are…
These are contained in the .IMAGESET file like this…
Obviously, the IMAGESET NAME, IMAGE FILE and IMAGE NAMEs all need to be unique to the icon set you are presenting to the game (you'd think not being able to find an icon is a minor thing, but it can actually cause the game to crash :P ), so go ahead and change those values for your new IMAGESET file.
For the X and Y coordinates I recommend (based on many hours of trying to make my own coordinates work and failing) to just clone an existing IMAGESET file and to not monkey around with the X and Y coordinates! :D This way you're sure it WILL work with the IMAGE FILE it's pointing at.
An infographic (I got lazy to write) about how coordinates work
So if you've cloned the IMAGESET file then you also want to clone the ICON SHEET it's pointing at, so that you preserve the layout of the artworks on the image. You can edit to your heart's content but you have to stay within the layout-box for each icon.
There is more detailed info about working with Icon Sheets here.
And you should be good to go!