If you are see something like “Your MobileMe account information is not correct. The provided login or password is not valid.”, a problem with old password in Keychain Access.

How to fix it:

1. Open “Keychain Access” and  search for “publish.me.com”, open it
2. Click “Show password” and change your password from old to new one.  Probably recently changed it.
3. Save changes.
4. Now go back to iPhoto and try update  MobileMe Gallery

P.S. Other way is  deleting publish.me.com from Keychain Access and then signing out and then back in to MobileMe in System Preferences.

Note:
This doesn’t really “fix” it but it has been a decent work around until Apple provides a fix. It wouldn’t hurt to let them know about the problem (select “Provide iPhoto Feedback” from the iPhoto menu).

iPhoto will not update MobileMe gallery is a post from: My MacOS X