wouldn't the relative path be ..//Images/foo.bmp? No variant of this that I have tried works including putting foo.bmp in the same folder as index.html and simply supplying foo.bmp, which the readme says you can do.
and Second, it would appear that you can't change the PIC_URL value to a relative path once the csv has been uploaded to AGOL and you are, for instance, trying to make this edit when in builder mode. Is that right?
Duh, it's working. I bet I wasn't saving the changes after importing the csv with the new relative paths. Sorry to trouble you, Thanks for the tip about bmps. I will convert them.