Yet another unlikely problem (nothing unusual there), but after installing Gallery3 via my host’s (VentraIP) Cpanel installer (Installatron), I got a 500 Internal Server error message.
After much Googling I didn’t find anything quite like my problem, yet I did notice a pattern: they all had to do with the php.ini or .htaccess files.
So the natural progression from there was to rename both the php.ini and .htaccess files to ensure that wasn’t the issue. Unfortunately I still got the 500 error.

Frustrated and without and further ideas, I discovered that I also had a php.ini and .htaccess file in the root directory (in which my Gallery folder resides). I renamed both and magically IT WORKED! So next I named the .htaccess back; still works.

And so then I knew the culprit was php.ini.
I went through all the entries commenting them out 1 by 1 and testing them until I came to the problem.
At the end of my ini file there was a zend_extension enabled that didn’t exist (probably a failed experiment from a long time ago).

Amazing that something so simple can bring down an entire web app.
Anyways, that’s fixed it; now I can add all my images (mostly wallpapers I’ve collected over the last few years, some of my own as well.