What would be the URL to display the WordPress image editor with an image from the media library?

If the edit link to your image is:

http://example.tld/wp-admin/post.php?post=551&action=edit

then the link to the image editor for that image is:

http://example.tld/wp-admin/post.php?post=551&action=edit&image-editor

The $_GET['image-editor'] is checked for in the edit_form_image_editor() function.

PS: I first noticed this trick, when testing this avatar plugin recently (I’m not related to that plugin)