Add Image can be done from the Lookup window. You can also Drag & Drop from File Explorer.

What's the source of the image when Paste is unavailable? It's a known problem for Chromium based browsers like Chrome.

Two questions regarding the Artwork window of the File Properties-tab.
Right click enables to select to [i]paste[/i], to [i]lookup image...[/i] and further options. However, sometimes the paste option is grayed out although the clipboard contains an image. Why?
Furthermore I miss an [i]add image[/i] option.