Screenshots are broken for Mayo entry

Hello, the screenshots for the Mayo application are not visible and appear broken at https://appimage.github.io/Mayo/

[appdir-lint.sh](https://github.com/AppImageCommunity/pkg2appimage/blob/master/appdir-lint.sh) did not report errors but just warnings:
````
WARNING: Icon is not in PNG format. It should be so that it can be used as a thumbnail
Validation was successful.
WARNING: Blacklisted file libX11-xcb.so.1 found
Lint found no fatal issues
````

What could be the cause of the problem?

* Link to the original application project's (upstream) download page from where AppImage can be downloaded: https://github.com/fougue/mayo/releases/tag/v0.8.0
* Link to the most recent AppImage for x86_64: https://github.com/fougue/mayo/releases/download/v0.8.0/Mayo-0.8.0-x86_64.AppImage