When I go to edit my signature, by adding an image, the button for insert image does not appear on the editing bar, even though the “add buttons” option says it should be there.
Is there a way to get the insert image button back to edit bar?
You are editing a signature synced from your server, and it doesn’t support adding images remotely. Therefore the toolbar button to do that is not visible.
Open the webmail interface for that address, and edit the signature there.