You said 'any help' ... so here goes:
'test run' ... how many students?
Quiz ... how many questions on a single page?
Check your servers error logs ... look for lines that were attempting to send the image to the students devices (what ever they were using).
Kinda sounds like server slightly over taxed and hickup'd on sending some images to student devices.
If debugging is turned on what does moodle report going to the quiz containing broken images.
Images are stored in moodledata/filedir/ ... a "sea" of files by non-humanly recognizable names and everything in there should have liberal permissions ... everyone can at least read.
So check ownerships/permissions recursively on moodledata/filedir/
Get the name (filename) of the image missing by viewing the question with the broken image.
You might see it like: mc2_2.gif
Copy that filename to notepad/textedit (text editor) - and use in a query of DB ... the 'filename.ext' place holder in query below
Table to query in DB mdl_files table:
select userid,author,contenthash,filename,contextid from `mdl_files` where (`filearea` like "questiontext" and filename like "filename.ext")
then using a tool to browse files (do not edit) find the contenthash named file
The userid, author in query will give hints/clues as to where one might find the quiz ... in what course/quiz.
Example of contenthash value ... and how to find in moodledata/filedir/
is located in moodledata/filedir/da/8a/
and the file is called da8a5b48bc6dae70c23850f7b2279a556c4ffeef
Can you view that file with anything you have to browse files?