While Mac OS X provides the core functionality of displaying images in Mail or iChat, it is limited to either pictures you manually have assigned in the AddressBook and to .Mac accounts. With MailPictures any email address can have an image and there is no need to manually assign an image to a person. So if I send an email to you my image will magically pop up in the email message. There is nothing you have to do besides having MailPictures installed.
Both sides must of course have it installed to work, as far as I understand. So make all your Mac friends install it. No, I don’t know what happens if you already have a picture assigned.