Thank you for bringing this to our attention. We have verified that there is a problem with the image selector when viewing a very large numbers of images. We're working hard to resolve this issue.
As an interim workaround, you can do the following: Upload all your new images to a new folder. Since this folder contains less images, you should be able to view all its contents on the image selector page. You can then add your new images to your listing.
I realize this workaround is less than ideal. We will work on getting this resolved as soon as we can. Thank you for your patience.