When I attempt to add an image (tried with png and gif), I get <!!image> and a newline. I waited for minutes (in case it was slow to upload) and disabled any blocking extensions (what didn't block anything in the first place), yet I still get <!!image>\n while attempting to upload with the built-in attachments feature.
<!!image>
Related: #4735
This is known and currently working by design, the idea is that the image is only uploaded when the ticket is created so pagure doesn't know what the path to the file will be, so it uses this <!!image> as a placeholder which is replaced with the correct links when the ticket is created.
On comment though the upload is live and thus the behaviour is different.