The Facebook profile photo for Fight Dirty Tybee gets your attention. It’s one word written on the beach. It’s not what you usually see in script drawn in the sand with a finger or stick. It’s not somebody’s name or “Peace” or “Love” or curvy lines that will wash away with the next tide.   The word is “Quit,” and it’s formed in block letters with piles of cigarette butts. Butts that were litter just hours before. Butts picked up by people who love the unspoiled beauty of the beach.   Every Sunday evening or Monday morning from spring to fall there is a beach clean-up hosted by the grass-roots group Tybee Clean Beach Volunteers (TCBV). They scour the beach for all the litter left behind by weekend crowds.   It’s straightforward work that is instantly rewarding. You make an immediate environmental and aesthetic difference. I joined the effort at the pavilion at 9:30 a.m. June 1.   Tim Arnold and the TCBV team were signing people in. Each volunteer got two pieces of equipment. That’s all you need — a bucket for one hand, a reacher/grabber for the other. The bucket has a painter’s cup hung inside it for cigarette butts; the rest of the bucket is for everything else.