It seems you're using a custom script (the icons of which are being pulled and used by this script), which means it could be pulling it in any number of ways. If the script you're using happened to be packaged with additional resources, they're likely being pulled from those resource files. However, more likely they're pulling from the iconset manually by script, meaning you're likely going to have to do some spelunking in the script editor to find out where and in what way.
Fortunately, as far as the default works-perfectly-fine-so-why-change-it commands go, "draw_icon" is used to... draw icons. The firstmost object in this functon call is the "index" of the icon used, usually pulled from the skill or item itself. In this case, chances are it's pulling a raw number. But which number?
Fortunately, VXace comes with a neat little cheat I only just now noticed:

Since "draw_icon" is structured in such a way that the index is pulled first, you can simply search through the code for "
draw_icon(116" in this case, and it should find at least one result, somewhere. Change the number, "116", to whatever icon you'd like to replace this, which you can reference either by counting icons on the list (topleftmost being 0, and each icon read left-to-right adding 1), or using the little box shown in the above screenshot.