Back in April, we published a hint
regarding seeing email senders' pictures in Mail.app. To make this work, though, you basically need to first email your image file to people you converse with, so that they can store it locally. Dominik Westner has now released a program known as MailPicturesBundle
that helps automate the process.
What MailPicturesBundle does is basically download your image from a public URL for other readers who have also installed MailPicturesBundle. This means that you can create your image and store it on your iDisk, and then anyone who has also installed the bundle will be able to see your image without any special action on your part.
The images are only downloaded once and then stored locally, so there's not a huge network hit every time you open someone's email. The program assumes an internet connection, so it may not work that well with dial-up access. It also relies on some undocumented Mail.app API's, so future updates will probably break it.
MailPictureBundles does not modify your source Mail.app in any way; the bundle is added to your ~/Library/Mail/Bundles folder, not the application itself. If the bundle gives you any problems, simply remove it and you'll be back to normal (at least, that's my understanding of how the bundles work!). In addition, the source code is available if you have any questions as to exactly what the program may be doing (assuming you can read the source, of course!).
MailPicturesBundle is an interesting concept and one which Apple will hopefully find some way to officially include in a future Mail.app release. It would be nice if we could (optionally) have the image side of Mail.app handled automatically.