Facebook computes a hash for every image hitting its servers. Not magic - known algorithmics (pHash, dHash, or their internal variants). If the same image shows up again and again - the system flags.
What this means in practice? If you upload the exact same JPG file with identical bytes 30 times - the algorithm sees a pattern. Account score drops.
What does the software do to avoid this? Before each Marketplace post it runs light-mod on the photo: a microscopic pixel-level shift, brightness, minimal noise. Human eye cannot tell. The hash changes. This is not AI and not redrawing - just a small mathematical edit.
What it is not? Not protection against human reports. If a user reports a stolen photo - no hash will help. Use your own photos.
Links: why we do not provide FB accounts, original photos only.