Literally it means "if you want/like". But we sometimes use that implicatively, like in the photo you sent. For example, a personnel has a laze, and another says to him "Bak müşteriler var, onlarla ilgilen istersen." It is not up to the personnel´s desire here, so this saying is not about the fact that someone will do something if he/she likes, but it has an implicative meaning laying in the deeper. Maybe, especially for some cases, we can say this saying is used as it has a polite tone to mask the direct request.