okay sorry for the delay anon! i mean it really depends on what you mean by putting a picture, but i imagine you literally mean adding a picture to the body of the fic itself. so for instance when i add a moodboard to a fic on ao3 i just go to rich text mode and lick on the image icon to add one. you first need to have your image uploaded into a image hosting site, meaning you need a link for your image. I host mine in postimages.org but some people use discord or tumblr or any number of actual image hosting sites. i hope this helps? ao3 has a lot of resources and tutorials on many things you can do on it (you can do some crazy things most of us dont even know its pretty cool) so just in case this was confusing here's a tutorial! https://archiveofourown.org/works/19017715/chapters/45163348