On the main "Bonk Images" section, there's a tab at the top labelled "BITS" that should allow you to adjust the images used for each bit type. You'll want to click the green "+" to open the file selector for that bit type!
I feel like the absolute dumbest person right now. I tried so much (even editing the JS myself) and I just didn't see that option before..Argh!Thank you for the fast reply!