If you copy straight from the e-mail, you have to remember that access to the e-mail is temporary in that it is routed through however many servers and gateways and into your machine. In other words, it's not a stable location that can be accessed from different locations.

To ensure it can be accessible, download the image somewhere on your machine that's easily accessible. Next, upload it to your account on imageshack, photobucket, Facebook, your profile or album on PC, etc... . Copy the URL, then once you're writing the message to whomever, click "Insert Image" and paste the URL. Additionally, by downloading the image to your computer, you can adjust the image in any way, whether it be cropping, changing something or saving it to a different file format.