Image Alt Text for Images not in Media Folder

Hi,

How do I add image alt text for images that aren't in the Media folder? I've installed a plugin, Ninja Popups, which imports loads of images, only a few of which I use. However they're stored in /wp-content/uploads/ninja-popups/ and just as an image file.

Images in the Media folder allow you to add Alt Text. Images in Ninja Popups are treated as any other file I guess. It could be a PDF or Word doc etc.

I feel if I was to move them from /ninja-popups/ to /content/uploads/2016/08/ for example then the plugin wouldn't be able to find them - and I'm not a coder so not keen on fiddling with php.

Thanks in advance.

Robin