Of course I was just speculating, but have seen enough really slick things with 2.1 I thought it might be in the realm of possibility. Should be a setting there though, x number of lines excluding images? I really haven't looked into it at all.
Seems kind of silly you can't hotlink a small image like the xf logo, in the sig all by itself.
FWIW, we just simply do not allow images in signatures here (that is demonstrated by the Insert image button being missing from the editor).
That means there's a couple of things we need to look into here. Firstly, you seem to be getting the wrong error message - it should be telling you images are not allowed, rather than saying stuff about the number of lines. Secondly, you seem to indicate that a small 16px image works. It shouldn't; images aren't allowed (can you double check you were allowed to set a small image in your signature @M@rc?)
One potential guess is that the image tags are being stripped (which is what we generally do in signatures with disallowed tags). That then turns it into a URL that gets autolinked. I guess that URL then gets turned into an attempted unfurl (which fails), but it counts as extra lines in the output.
There's a separate bug about not unfurling in signatures, so if we change that, it would probably resolve this anyway.