You probably have your image slots full. Try to delete the images you don't need and try again (if you only have that OF and didn't import anything else you can even delete them all, since they will all be re-imported).
If that doesn't work, check if you have too many images in 2048x2048 (large size). I recommend you install most (if not all) of the images in 1024x1024 (small size), in game you won't notice much of a difference anyway.