Usually if you want to be really nice you follow them back, just click on their picture in your "followed by" which will take you to their profile. You *can* go post a comment (not an answer please as it would be off-topic) to something they asked or one of their Answers saying HI, thankyou and I followed you back if you like.
"Bananas... in pajamas... are coming down the stairs, Bananas... in pajamas... are coming down in pairs... lalalalalaaaa...." LOVE your pic! If they're following you, chances are good that they will comment on one of your answers... just comment back, and voila! A "friendship" is born!