If possible, the screenshot should be uploaded to https://cgit.kde.org/websites/product-screenshots.git/

See https://cgit.kde.org/websites/product-screenshots.git/tree/README.md

Just a minor thing: Since this is a new metainfo file, you might want to give the project the component-id org.kde.filelight and add a <launchable/> tag as described here https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-launchable and in the quickstart guide: https://www.freedesktop.org/software/appstream/docs/chap-Quickstart.html#sect-Quickstart-DesktopApps

In this case, the tag would be <launchable type="desktop-id">org.kde.filelight.desktop</launchable>
This would be very "modern" AppStream (the feature is about 6months old) - the file you already have is valid too, of course.