Install the Tastelint GitHub App. It reviews screenshots on pull requests and comments as Tastelint.
Opened, ready-for-review, synchronize, edited, and reopened on a pull request. Edited is how screenshots that land in the body after open get a review. A comment that is exactly /tastelint re-runs the review. Drafts are reviewed too — that is when the screenshots usually land.
$40 per designer each month, 25 reviews included, $2 after. Antiwork repos we already run stay free. Everyone else subscribes on the pricing page — the app will link it if a review arrives unpaid.