This problem can occur if a message has been forwarded back and forth where one or more of the persons forward the message are using rich text and someone else on the chain is using plain text. Once a person replies or forwards the message using a plain text email screen, the image references will become unusable. Please note this will occur with any email system once one of the recipients replies using Plain Text as opposed to Rich Text.
This can also occur from the repeated sending of the message. After being forwarded many times, the images themselves can become corrupt.
If you still wish to send forwarded images, we suggest that you download the picture to your computer, compose a new message and attach a fresh copy of the image so that any messages you forward will be in Rich Text. However, please note that if someone forwards that message again using Plain Text, the problem will resurface.