Do you have a large number of photos uploaded?
There's a bug they're working on right now that's making it so it the Select Image box won't show all of your images (if you have quite a lot in there).
However, if that is the problem you're having then a workaround is to break up the images into more than one folder. I don't know what he magic number is but if you go into a folder with fewer images you might have better luck at having them show up.