I know, this as an old thread but I actually had the same problem, that "Simple Image Gallery Pro" pop-ups where rendered as jpg in a new tab instead of rendering them in a gallery.
In my case I could resolve it by deactivating a templates setting.
My template is
John version 3.19 / Joomla 3.4.8
In the template settings I had to set
Advanced settings -> JavaScript compression to
DisabledThis worked on my live website as well as on my local webserver. Could you please check, if there is an error in this template feature and fix it, if there should be further versions of the template?
Now galleries work again as you can proof here:
http://www.mediaventa.de/themen/fotografie/item/305-kapstadt-2013Best regards
Roland