Please provide more detail, as this does not seem to relate to Thunderbird. As I understand the steps, - you receive a jpg from a gmail account and store the jpg on your computer. do you then open the jpg from hard drive to validate? - next, you compose a message and attach the jpg to the message and send - then, the recipients don't receive the attachment.

Is the attachment in your sent folder? have you tried sending message with that attachment to yourself? With more info, some suggestions may be offered, but this can also happen if email provider or antivirus is involved. thank you