Gallery Issue

I’ve just more or less got my site up and going. So far all my issues I’ve been able to resolve on my own, but this one I’m having trouble with. Any galleries I’ve set up work fine, but if I want to add any pictures to the existing gallery, I’m having some trouble. I can upload a new picture, and it updated the gallery no problem, but if I want to add a picture from my “Media Library”, I just can’t seem to do it. Am I missing something?