Based upon my testing of this issue, it is because Mail (also Thunderbird) attachs with a header that includes that it is Inline. Lotus Notes cannot render these correctly, and Notes developers are aware of the issue, and perhaps the latest version handle, but I do not know.
The irony is that they can Forward these messages to others not using Lotus Notes, and the message will still be in good order.
If you zip the JPEGs in folder, then I think the images will get through OK, but they will have to unzip the folder.
Ernie