Let's take https://beta.musicbrainz.org/release/742a1ade-7364-4017-ac5d-0ef8956e05b4/cover-art as an example:
- Click the first image
- Click next
- Everything's as expected
- Click previous
- Image is still #2, text is "Image 1 of 4"
- Click next again
- Now we're back at "Image 2 of 4"
This happens on not-beta as well
This happens in Chromium 31.0.1650.63 (238485) and Firefox 26.