No screenshots in appdata
It seems that the previous screenshot was removed in commit 16b1cfb9 because it was not up to the mark and doubts on how to maintain them. As far as I know, there isn't any automated harness for the screenshots yet. Most GNOME applications have been manually maintaining them. Here are some guidelines:
Some people have been hosting screenshots on https://static.gnome.org/appdata/ through the Infrastructure/static-web module because it's better suited for serving static content, while others put them in the application's Git repository itself.
Pro-tip: I have learnt that distributors often aggressively cache the screenshots on their infrastructure. So it's better to use unique file names when the screenshots are updated in future, instead of replacing the old file and reusing the same file name. Otherwise, Software continues to show the old image.