Bring a Trailer is unusual among listing sites because it publishes both ends of its acceptable range. Photos under 1 MB may fail to upload, and bidders expect roughly 2000-3000 pixels on the long side, which lands at 1-4 MB per photo; the uploader caps each file at 10 MB. The most common upload errors BaT lists are, in their own words, photos that are too small, photos that are too large, and incorrect file type — and HEIC from an iPhone is explicitly not supported by the uploader. That makes the failure mode two-sided. Over-compress and the upload bounces; leave a modern phone or camera file alone and it may exceed the cap; shoot on an iPhone in default settings and the file may not be accepted at all. A typical first submission is 10-20 photos, so a 100-photo documentation set gets built up across rounds and every round hits the same limits. The reliable sequence: crop to the ratio you want, resize so the long side lands near 2500px, compress until the readout sits around 2-4 MB, and export as JPG. Skwish does all of it in the browser with a per-file before/after readout and a batch queue, so a folder of 40 goes through in one pass and comes back as a ZIP. Files never leave your machine.