all I see when I do this is the mod version number. I'm not sure about OP but I am looking for the project ID used for the manifest.json when submitting a modpack. :)
This means that the picture needs to be a square. I use canva https://www.canva.com/ which allows you to choose your own dimensions.
If you have a picture already you can try resizing it here https://www.canva.com/