First a suggestion: Please allow multi-image upload. Because if not, a problem arises.
Which leads me to second, after I load up 'x' number of images, the 'add image' box goes off the screen! The more I add, the lower on the screen that 'add image' box gets.
I think that if I 'fiddle' around with up.down on the page, the add box is accessible but as you can see, after 12 or 13 images, it becomes nearly unusable.