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You can flip through your existing photos in typical iPhone finger-gesture fashion or take new ones with your phone and upload them to the MobileMe cloud. There's an option to send photos both new and old to friends and colleagues via e-mail, and the app views videos, too.
You can also use MobileMe Gallery to browse friends' photos; any friend you're following will have a blue dot next to his or her name when he or she has uploaded new photos. Chances are you're sharing your photos on Facebook, Flickr or TwitPic directly because not all your friends have Macs or iPhones, but the Gallery app could be helpful for keeping up with friends if you do hang with a bunch of other Mac enthusiasts.