If you google that message you find it's been discussed to death. I didn't offer more than this because I'm unsure which exact email software and version you are using.
The most common workaround is not to forward but to create a new message and copy/paste the content.
Bob
When I attempt to forward emails that contain pictures I sometimes get this message: "One or more of the pictures in this message could not be found. When you send this message these pictures will not be included. Are you sure you want to send this message". Anyone know why this happens and how to resolve it or have a work around?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic