There’s a new version of the AppData validation tool, and this one actually downloads and validates the screenshots you’ve included in the .appdata.xml files. See the AppData specifications for guidelines on how to make good screenshots.
Please yell if you think you’ve written a valid file, and it fails to validate using the tool. We’ve added a lot more checks, and we’re getting more strict — so please revalidate your file if you’ve already done so. Thanks!