Well, if you were interested in manually editing the attachment.php page for one graphic, you could do something like the following in that attachment.php page:
“< a href=”yourblog.com” > img src=”/yourpic.jpg” < /a >”
With this solution though, you’d need to edit the attachment.php page manually, and you’d need a fresh attachment page for each image you use.
If you are just interested in opening the image full size in a new window you might want to check out some of the image plugins available. I’m using Vivtiger Image Resizer to allow users to resize their thumbnails. It has options to link to larger images, and whether you want those links to open in new windows. This would allow the user to close the full-size image window and thus return to your blog once they are done viewing.