Aha, Libreoffice Draw lets one put callouts on an image, so at least there's *something* for Ubuntu which can do it. Pretty clunky, though; I wish there were another app that could do this that was nicer to use. (And defining new callouts for Libreoffice Draw to add to its anaemic collection seems to be very hard and un(der)documented.) Small project for someone, perhaps me. https://mastodon.social/media/Xi084unamndJuK6hqjc
(also, that *were* nicer to use, sorry)
Follow friends and discover new ones. Publish anything you want: links, pictures, text, video. This server is run by the main developers of the Mastodon project. Everyone is welcome as long as you follow our code of conduct!