For me, this problem was not caused by permissions.
Instead, it was in the .htaccess file. It was either +FollowSymLinks, or a couple of lines of RewriteEngine commands (as part of a requirement for a wordpress plugin in the same htaccess file).
This DEFINITELY was the problem on my system and I tested with and without the extra lines a number of times. Re-adding the commands stopped the images working every time.
Before editing htaccess, images were not loading and were not being mapped as a resource for categories or products through the resource library manager. Many images would not upload even with the upload button.
Also, images which were eventually linked as media, would not show up full size when hovering over the small product thumbnails with the mouse. The image only came up with an empty box and said: "loading ....". But nothing ever loaded.
Anyhow, start by looking at .htaccess file and delete sections part by part to test.
Hope this helps.