I can do so.
Go to "Signature" and paste the BBCode for the image tag, now, that image must exist in Kiwifarms I believe, or its complete link must be of public access (like not in a DM chain).
So essentially, try putting this in your "Signature":
[IMG]https://kiwifarms.st/attachments/mystery-png.7195470/[/IMG]
Try it and tell me if it works. Now all you have to do is replace the link you see inside (the one above) with the one from your image, and this image must be uploaded on Kiwifarms; it's also of public access I believe when the name is displayed (in the link) & not originally uploaded on a restricted part (like a DM).
Something like that, I have not tested all the different combinations to see how Xenforo works, but anyways, try my link first and tell me if it works. All you have to do is copy-paste the [IMG] tag up there.