It works with a smaller (450KB) image ~

There is a correlation with file size. The picture I tried to upload was particularly big - about 20MB I think - so I probably should have downsized it anyway. There was no problem with uploading the few KB big screenshot.

Interesting, I did not know about pict-rs. Thank you, I will just try again, maybe with a downsized version of the picture.

Inside the home even.