During the recent successful search, the searchers trawled 4chan archives for early-2010s posts with similar image metadata to the 2018 Backrooms image copy. These archives were missing the original image files and thumbnails, but still retained some image metadata that could be filtered on (dimensions, image file md5s etc.) One of the searchers came up with a list of posts which might have originally included the image file, based on image metadata and context. Another searcher plugged the image md5 of one of these candidate posts (an April 2011 post recently added to an archive) into other archives, and hit on a post with a thumbnail matching the original Backrooms image from March 2011. At this point they'd finally found an earlier copy of the image, after years of searching.
Soon after, one of the searchers plugged the filename of the March 2011 post into Twitter's search, and came up with a post from 2019 which included the physical address and a link to the image source (this Twitter user had already found the source before the search had really begun, but it had gone unremarked upon at the time). The website had been replaced with blogspam in the interim. A searcher plugged this domain into waybackmachine and found a page with the image and a full explanation (it was taken during the renovation of a commercial property in Wisconsin).
Post from one of the searchers here: https://www.reddit.com/r/backrooms/comments/1d3pkif/how_the_...