There is now a "latest" collection (or "latest_nsfw").
The feature didn't look much used, since it didn't work on many
backends.
Using collections will make it easy to support other things
like most viewed, featured, etc.
As a bonus, I added tests for every backend with the feature.
It is now in "data-src" instead of "src", because they use JavaScript
lazy loading.
Also ask for the bigger thumbnail, and remove useless anti-caching.
