While stores and film processors are busy waging price wars over rentals of videocassette movies, Montgomery and Prince George's County libraries are lending them at the cheapest prices in town. Maryland residents may visit a Montgomery County library, give three weeks' notice and obtain any of more than 80 films (both Beta and VHS formats) free of charge, just by showing a library card. The films are on one-day or weekend loans. Prince George's County libraries also take reservations for and lend Beta- and VHS-format videocassettes, but they're charging $2. Their collection includes "Coal Miner's Daughter," "Alien" and "10". The Prince George's County library system also boasts the country's first on-loan library collection of videodiscs, of which they have about 20. The bad news is that the rules which allow District and Virginia residents to borrow books from Maryland libraries don't apply to videocassettes or videodiscs . . . The Silver Image recently introduced the instant portrait T-shirt. Visit the shop, have a Polaroid taken of yourself, and leave, 15 minutes later, with your face, in 8x10 color, on a shirt. The secret involves a photocopying machine and heat. The $25 fee includes the shirt and photo. The process also works on other materials. Call first. (3203 Grace St. NW. 337-3705.)