Now I understand the issue. We do not support HEIC format. The good news is that Apple auto converts to JPEG when you drag and drop the images from the photos app to the desktop or into our uploader. This is on Catalina 10.15. I am not sure about earlier versions of the macOS.
of the earlier versions of the MacOS. I really had not namely any difficulties inserting images when having earlier versions. Problems just started occurring after getting the new version. As well it is good to know that the images are converted automatically to JPEG when dragging and dropping them to the desktop.