Rule 3.f. Signatures.
Signatures may not exceed 450 pixels in total height.
This means that you have 450 pixels to fill with pretty much anything (that follows normal forum rules)
It does not matter if you have 10 images or more or if you have 1 image and text etc. as long as the sum of the total height is no larger than 450px.
You can use [Spoiler] tags in your sigs if you want and they won't hold any restrictions as to height because users have to manually click the Spoiler.
The reason behind this setup for the rule, is that all other setups are hard to enforce and can be bypassed easily.
And nothing hasn't been done about that...congrats!