I have this problem too, and find it a downgrade from the old image viewer.
I noticed all the work you put into the new image viewer, and really like how you can browse back on the last uploads right on the upload form. It makes posting the same picture again really easy. Thanks for that.
But this new viewer has this problem where you can't close the image by just clicking on the background.
- Go to http://www.head-fi.org/t/529140/show-us-your-head-fi-station-at-its-current-state-no-old-pictures-please/8880#post_8750795
- Click on the first image in the post. This brings up the lightbox image viewer, and shades the background.
- Click on the page in the background.
The background unshades, but the lightbox viewer with the image stays there.
The background unshades, and the lightbox viewer with the image closes.
Note that before the modifications to the image system, clicking on the background closed the viewer. Now we have to click on the [X] at the bottom left of the viewer to close it, or press Esc.