You'll need a central location with plenty of room for costume changes, picture-taking backdrops, and a waiting area. The best times are usually Saturdays. Pre-sell the event with flyers and "reserved session" tickets.
You'll want to get photo commitments up front from 250 people to make this worth your while.

Your sales pitch should "Focus on the Fun":
1- Capture The Moment (before it slips away)
2- Dress up Picture Party (be there or be square)
3- Goofy Faces Wanted! (yours included)
Or tug at the heart:
1- When's the last time you sent a family photo Christmas card?
2- They're not getting any younger... Get a portrait!
3- Mother/daughter, Father/son - Pictures last a lifetime!

Your group can coordinate with a local photographer or partner with a national photography chain. One that I recommend is Vista Studios. They offer a 10x13 family portrait for $8 and your group gets paid $6, plus additional performance bonuses.

Action Shots
Another great picture fundraising idea that works especially well for sports teams is selling candid shots of a playoff game or other high traffic event. Don't forget to get shots of the spectators too!

Ask someone with the team with camera equipment and a love of photography to take action photos during the game, award ceremony, or even season kick-off practice. Sell the photos to parents or participants for a fee that goes to the team funds.

Remember that your photographer may have expenses that will have to be reimbursed, but the cost should be nominal.

Use a collection of the pictures to create a team collage photo.

For any photo fundraiser, you can add to your take if you offer photo buttons, photo key chains, photo mugs, and so on. While it certainly makes the organization of the fundraiser more complicated, you can also add to the bottom line by offering a variety of photo options.

Having these items made can be done locally, or in the digital age, upload your digital pictures to a wide variety of companies that offer this service. Always get pre-paid orders before purchasing any of these items since they are usually non-returnable unless they are defective.

Combine a Get the Picture fundraiser with another fundraising event. Whether your fundraising event is a Spring Carnival or Casino Night, make sure that selling candid photos is part of your fundraising picture.