Indeed, many popups don't close by clicking on the image itself. They usually have an X or "close" button somewhere to close them.

I would recommend using PrettyBox or FancyBox which by default auto-resize the image (with HTML) to fit the browser window. So even if you're on an iPad, you can browse the images like on desktop PCs. Check this page demo.joomlaworks.net/simple-image-gallery-pro with your iPad.

For mobile phones with smaller screens compared to the iPad or other tablet devices, as you can understand, popups are not the best way to show your images. I would recommend using the "Galleria" template there which basically previews the larger image "in-place" (although if you click the large image area the popup still works - but it's easy to cut it off from the Galleria template).