I would like to show a picture/drawing embedded in a message (in addition to as an attachment). The picture/drawing is on my pc as a .gif-file, I do not have website with an url for it.
I have seen some messages display an attachment picture embedded in the text.
How does one do that?

just upload the file as an attachment to the message. Then go back and edit your message. You can then cut and paste the URL to the image uploaded to catsailor.com in your message.