Freedom of speech is related also to the freedom to read. "Your freedom ends where my freedom starts" my dad used to say.
A flashing "BIGGEST CASINO ON THE WEB" pop-up, for instance, might be freedom of speech for some but the fact that it pops up in front of my face without me asking for it is coercion.
I'm actually not against someone having a political note or message written on the signature. However, a distracting and gigantic animated GIF is just lame.
It does not matter if the message is political or not - the way the signature is being put there is what bothers me. You look at it once and it's funny. After that, it becomes a pain in the ass.
So you want to have a signature that expresses your beliefs, your tastes, yourself or expresses nothing? Cool. Just don't make it so distracting that it makes reading the forums a pain. Use your common sense.