Hi Kyle,
I think I found something about my issue. To check the origin of 503 errors from serveur on most of images i made a local install of Joomla-CB with XAMP and i had the same issue ( worse in fact !) but i found this in the serveur log
Fatal error: Allowed memory size of 134217728 bytes exhausted (tried to allocate 20480 bytes) in C:\wamp64\www\cpn\libraries\CBLib\Imagine\Gd\Image.php on line 763
In CB main configuration, the setting Image Software was on "Auto". First I set on "Imagick" and i works perfectly now ! The weird thing is it is also working perfectly if I set on "GD2" . Is "Auto" trig something special ?