Re: Need help understanding signatures

For me, the signature feature is very unintuitive. I've been here for 3 weeks and only just learned how to edit it last night (I'm on phone so I only have icons, not sure if the desktop version uses words in the settings tab). 
Right now I can't figure out how to get a picture of my novel's cover to show up in my signature. Is this a premium-only feature?

Also is there a setting I can hit that auto-checks the "Display your signature under your post?" Box?