Okay, I'm officially blind. There's the -I option, which I haven't found, because I search for icons, not for images. Maybe adapt the man page to let people like me find it easier? sorry...

The issue with changing the manpages (from my perspective at least) is that flag doesn't allow "icons" but specifically images. The images you see in drun are just automatically found for you, but you can embed custom images into dmenu mode. I used it to make a wallpaper picker.

The problem I have with changing the terminology is as ~cloudninja pointed out. It technically is not for icons, any mode can render any images it wants. drun happens to render application icons but that's by no means all that flag allows for.