The Image Pickin' forum started out as an idea in Trope Talk. I don't recall what exactly the motivation was for the first post, but I'm sure it was something along the lines of "some of the page images on the wiki are really crummy. Should we do something about it?" From there a long-running thread emerged. This thread consisted of several hundred posts about tropers just discussing page images and their possible replacements. Those who remembered it was there would simply make a new post and request input on action, and through this, the thread lasted many months.
A lot of good work came out of that thread. Pages with poor images brought up in the thread only had about a fifty-fifty chance of actually getting changed, but this was a pretty good ratio, especially compared to Trope Repair Shop. The thread started before the Trope Renames forum was merged into Trope Repair Shop. This, coupled with the wide perception of page images as not being a serious enough issue to mandate documented consensus, was what led to the establishment of Image Pickin' as a forum in its own right. I don't know who made the request or if any request as made at all, but one day the thread was locked and a new forum appeared. I don't recall whether it was called Image Pickin' initially, but that's the only name I can remember, and it's a good one, too.
I had come to see the image selection thread as being a bit of a respite for the sometimes exhausting work I did in Trope Repair Shop, as the topic of interest was gleefully subjective and comparably low-stakes. For this reason the instructional stickied thread I prepared for Image Pickin' was quite different from the detailed argumentative explanation that was When To Rename A Trope. I pretty much just explained how the crowners worked, and what makes for a good page image- basically, the image has to demonstrate the trope in action. People may disagree on what constitutes an image of a trope in action, but these disputes were the issues this forum was intended to address.
Now, this was all just my general opinion. Others did make an effort to demonstrate clearer image standards, most notably with Just A Face And A Caption, which laid out how an image was a failure if it just consisted of a character (usually from Anime) looking at the viewer with a nondescript expression and a caption underneath the picture explaining their thoughts. The general operating principle was that a picture that requires a caption to make any sense fails as a picture- there has to be some useful visual context or else it made more sense to use the phrase as a page quote instead. I had nothing to do with this guideline, and only found out about it when Just A Face And A Caption page itself became the subject of an Image Pickin' topic, as to what face was truly the most generic and useless without context (they settled on Haruhi Suzumiya, a character with a large fandom and a strong personality that is not at all apparent from the subdued picture). It's a good guideline, although there were always lingering difficulties explaining it to some people- one problem was that some interpreted it to literally only apply to faces, when it was quite a bit broader than that.
Image Pickin' was one of my favorite parts of the wiki, largely because even though there were plenty of reasons for it to get derailed into the Serious Business that was damaging Trope Repair Shop, it never happened. People were much more certain in their opinions that an image was bad, and even those who liked the substandard images would admit that an improvement was possible. The environment was so laid back I don't know if Image Pickin has ever even had a controversial debate. On one occasion I asked for help with a replacement image for Fat Bastard, a trope which I had created and is pretty much what it sounds like. When another troper provided me with a picture of the actual character Fat Bastard I replaced it immediately just to see if anyone would object. No one did, and the topic was closed.
Another lingering issue that helped this perception along was the fact that page images had always been added and maintained by regular users. Creating any sort of bureaucracy behind it seemed silly when casual tropers would often change images of their own volition. In one memorable instance a poor page image was changed while it was being discussed in Image Pickin' by a random troper who had no idea said discussion was going on at all. The new image was an improvement, so no one really pressed the issue.
When all of these factors are taken together, Image Pickin' was, at least the last time I saw it, the one part of the wiki that had a positive influence on page maintenance while still maintaining the cool, breezy atmosphere that people expected and enjoyed in the regular part of the wiki. Because of its progress, I made a mental note that asking people to be nice to each other on the wiki could actually work pretty well. At least if you're doing it right.