Hopefully, this should convert any .svg, .ico or .icns files you might manually place in the Icons Folder to .png. It may not work for all files, as I'm using limited built-in macOS tools (for compatibility).
My intention was to stick with .png files, but this is probably a worthwhile addition that shouldn't slow things down too much.
I found what's causing the problem for me and it's spelled out in this thread:
Basically I'm stuffed because all my user files are on a removable external SSD and the Custom HTML Prompt can't load/display such files as WebKit won't allow it and there doesn't seem to be a way around it.
If I'd been aware of this limitation maybe I wouldn't have moved my user files in the first place!