I've been experimenting with this, and it took me a while to figure this out using built in icons.
I used this to find the gateway address:
print system.util.getGatewayAddress() and it returned: http://localhost:8088,
so naturally, taking the original working code from @Corne_de_Jonge combined with @PGriffith's suggestion, I tried: <html><img src='file:http://localhost:8088/system/images/Builtin/icons/24/check2.png'/>,
but that does not work!
file has to be changed to url, so the final and working code looks like this:
<html><img src='url:http://localhost:8088/system/images/Builtin/icons/24/check2.png'/>
The preceding code on a button's Mouseover Text property produces this result:
