GIMP :: Won't Recognize Images That Downloaded Using Nikon Transfer

GIMP won't recognize images I downloaded using Nikon Transfer. They are in the Pictures library and a Nikon Transfer file shows with everything else when I click File Open, but I can't bring them up to work on in a normal way. I was able to physically move one of the images, listed as DSC, from Nikon Transfer to the Gimp window and modify it normally, but the image was very tiny in a full-screen Gimp window, (which lately I can't permanently get back to the normal size). Saved the image as xcf and jpg and placed in a new file, but still can't work on it.