It may be just a case of working out on your phone where the photos are stored when you are adding an image to XFMG. On mine when I click on Choose file in the Gallery, I get an overlay on the phone which says "Choose an action". There are 4 options (Android System, Video camera, sound recorder and Documents). It's Documents that I choose to then be able to go to the folder with my images in.
Of course it will be different for different phones but it can definitely be done.