First, you must have somewere you can upload the image to.... A host...
Alot of people use Villagephoto's (
http://www.villagephotos.com/ )
Then
Using the IMG button above the text box you use to print.... you place the URL (address) of the image at your host into this message box that will appear....
Hit ok... and it should display....
Unfortuneatly... DH i doubt would be able to support the load and overall ammount of images if they were to allow File/Image Attachments.... the space and bandwidth consumed would be outragious....
If you need more help... just ask.... i'm sure we can provide more...