1. Convert your image with some Base64 encoder.
2. Paste it on Bitmessage and add instructions to the recipient how to view the image.
If you want to view this image, you must click on the message (in the Inbox list) with the right mouse button and choose "View HTML code as formatted text".
3. The recipient must read the instructions and follow them in order to view the image.
The current version of Bitmessage supports these HTML tags.