When I sent myself an email with inline picture using TheBat and GMail. I receive an email which shows the picture perfectly.
If I send the same to a friend using her Gmail address and receive a reply the inline picture only show an empty frame, and the picture is added twice as an attachment.
The top-right corner, next to the From, To, Subject field shows an ixon witha green down error. I guess that's a download button. Nothing happens when clicking it and I want the pictures always displayed (from this person).
First, this is probably something that you should report directly to Ritlabs using the Support menu above (and when you do, please include an exported copy of that troublesome message).
Secondly, the editor that produced the message is Gmail, if I understand correctly. So, maybe it's not The Bat that is at fault.
You could try sending a message from and to your own Gmail address in The Bat. Then, still in The Bat, look in Sent mail (not Inbox) when the message has arrived. Reply to it, wait for the reply to arrive, and see if it has the same problem. If it does not, then maybe Gmail is to blame instead of The Bat.
I volunteer as a moderator to help keep the forum tidy. I do not work for Ritlabs SRL.
My first approach would be to check what the friend did. All we know is she replied. Was she using the Gmail web interface or another email client? Did she intentionally (or accidentally) make a second copy? I don't see duplicating this, as we don't know what she did. Or am I missing something? Just my two cents...