When I initially evaluated this lab, I googled around to see if anyone had this similar behavior. Interestingly, I did find a thread here.
Once implementing the onload handler at run-time, I noticed that following changing the src of the image, the onload handler would not fire. Initially I thought this was related to caching (as many others mentioned the same thing on various forums). But I ended up clearing caches and even went to the extend of adding GET parameters to the photo URL to work around potential caching issues. Unfortunately, it had not solved anything.
The standard behavior of the test across all browsers (FireFox, Opera, Chrome, Safari) seems to all replicate the same issue. There were odd times where FireFox did manage to properly load it on a cold-load, but following a refresh, replicated the behavior of the other browsers.
You can find my example here: https://st3v3n-d.github.io/html_img_onload_debug/