Hi, I would like to point out that this problem is not exclusive to quizes.
I am editing the summary on the home page and am having the same problem. I see there is a tracker for the problem/request for change. Will try to add my information to it (Moodle-forum and Tracker-site is new to me).
 Ow... I see the tracker is closed already because it wasn't a bug.
But the resolution is not a fix for my problem.
For now I will workaround it by uploading the image I want in another part of my site, and then use the url of it in the text editor of the block on my homepage.