He is using hyperlink images in all of his posts and I can also assume that this is the reason why he does that. It may not be ethical but it's a clever workaround. Is this allowed? What's the applicable rule if not/
No. It's not allowed[1].
It's not a clever workaround. It's just the user asking to be banned. Signature restrictions are there for a reason and attempting to circumvent said rules obviously would warrant a ban.