I have installed Simple Image Gallery, but it does not show the pics in pop-ups, it open them in new windows. I have tried using Easy jquery. It solves this issue, but messes up many other extensions I am using, so this is not an option for me. How can it be solved? Thanks.

@zaruc

Did you read the previous post? Have you tried to set the "Do not load the jQuery library" through the plugin parameters? If this still doesn't work for you you will have to ensure that your site is loading jQuery only once.

I have tried setting it on "Do not load the jQuery library" through the plugin parameters, but it still does not work. How can I ensure that the site is loading jQuery only once?

This is up to you. You need to track all the extensions you have installed that are loading jQuery. Then setup them properly. An other option can be to load jQuery in your template and disable it from other extensions. Alternatively there is also a plugin for handling jQuery loading named "jQuery easy".