Hi,
Your question was about removing the popup altogether while changing the on click on the thumbnails to change the main image.
That's not possible without development and I was mainly answering to that question of yours in my message.
Now, the popup not displaying properly is a problem with the popup system used to display the image.
Looking at your screenshot, it seems that in the "image popup mode" setting, you've selected "mootools" which is an old popup system from 10 years, not well adapted to smartphones.
The default mode is "shadowbox (external)" and it shouldn't have the problem you're describing. If it does on your website, we would be happy to have a look at the situation if you could provide a link to a page with the issue.
Also as I said, the plugin provided by obsidev on our marketplace offers more popup alternatives, with some of them even optimized for smartphones. So you might want to look at it.