How to copy image url from thunderbird, when I try i get something begining with imap://myemailaddress@mydomain?part=...&filename=

I'd try to copy the image, or save it as a file, then paste from the clipboard or drag-and-drop into the new message. The link tells you that the image itself is encoded into the original message, so you need to disentangle that image from its host email message. It isn't just a plain old <img src="..." > html link to a file on a webserver. I don't know why these things don't "just work"; you ought to be able to just select the original message, then use forward, edit-as-new, or copy-and-paste its content into a new message, but Thunderbird does have problems with some graphic content. And some servers or email systems give trouble too. I get unreadable attachments (it usually says they're empty, but there clearly is binary content) in messages that arrive via outlook.com/MSExchange, but the same messages coming by a different route via gmail and landing in Thunderbird are just fine. Go figure. :-(