Same issue for me too. My boss is telling me to stop using Airmail. It doesn't appear when I'm sending it, nor does it appear when they reply. But when I reply to them it definitely creates weird attachments of email signature images. If the recipient is in Gmail, it does NOT show paperclip attachment but it DOES show actual attachments.
I've tried changing my email signature images to be HTML links, but Airmail will still create them as attachments when I reply to a reply.
Closely documented and asked the dev team twice, but they don't seem to be interested in fixing it. Not surprising given that Airmail isn't exactly expensive. I would really want Airmail app to become subscription service and just be more responsive to all these bugs.. great design, many bugs, and bad composer usability.