The gallery must have the option to "Add photo by e-mail" turned on. (You can change this when logged in to
www.me.com. You CAN'T change the settings in iPhoto. Once you've made it there, iPhoto won't let you access the options again.)
That's how the iPhone does it. It e-mails it after you click the "Send to MobileMe" button. When you click that it looks at your gallery and shows you any albums that have the option to send by e-mail.
Also, I had trouble where I could send to albums on my main gallery page but I COULDN'T send to an album that was hidden from the main page. I think that's a bug as I did do it once, but then that album kept refusing to show up.
Just something else to keep in mind.