Thank you so much; it works!
For anyone else experiencing this problem: TIFF images work in the PsychoPy Builder offline, but do not run online. We had to convert our images to PNG; we used GIMP image manipulator to do this by using the ‘export’ button, but importantly had to ensure that only the following settings were ticked:
Here is a thread that helped us, too: IOError: Found file but it failed to load as an image - #2 by kevinhroberts