Hi Julian,
Thanks for posting about this here. I'm afraid I don't have any good leads as to why that has started happening on your end because I am unable to reproduce the behavior and have not received any similar reports from other customers.
When I click one of the "Select an Image" boxes on the Auctiva listing creation page, there are actually 2 blue "Done" buttons. One of them is in the upper right corner of the pop up window and the other is centered on the bottom of the page. I am able to see the first one without any further action but I do have to scroll down past the thumbnails to see the one at the bottom of the page.
I also considered the possibility that you may be referring to the "Insert an Auctiva Hosted Image" button on the description editor toolbar but ended up dismissing that idea because there is no "Done" button on the page with the thumbnails.
I will create a support case for you in a moment and invite you to send me a screen shot of the behavior you are seeing and hopefully we'll be able to get to the bottom of this. You should receive that message at the email address on file with your Auctiva account.
-Mike