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I find the borders ugly. Maybe the shade in the resolution bar can do this, leaving the icon to represent resolution.

An approach would be to use the number of posts in the hour surrounding the post. The shade could vary based on this number, for example.

This has the advantage of being generic; posters don't have to do anything new. Not all batch posts are appropriate to pool (those MT images that are often otherwise unrelated); not all pool posts are batches (single posts that are added to older pools); there's no need to fight to get people to change their habits to support any new mechanism.

If someone posts a 50-post batch, and then immediately posts a separate image, it's indistinguishable from the batch. That's fine; the goal is to be able to see other individual posts among someone who's posting a lot, not to try to tell the exact boundaries of a batch.

A very different approach: on thumb mouseover, highlight (in whatever way) all posts on the index page by the same poster. Works for all posts (not just "batches"), simpler to implement, but it might be annoying. I might give this an experimental try, just since this is a 10-minute Javascript change instead of a few hours of Ruby (and I'm not sure the rare large batch posts are enough for me to invest that much time in this right now).