How do you put those custom animations on every post you make?

It's your signature;

Go to your name in the top right hand of this page, and click "Signature".
After that, add a .GIF (the animated thingys) to your signature.
Tada, you're done.

 

Here it is.
See this in your top right hand corner (not fan312, it should be your username)
Click on your username, and this should pop up.
Like I said, it should be your info.
See signature?
Click it.
This should pop up.

Place the .GIF into that text box area, like where mine is.
Press "Save Changes", and you're done!
Tada, you have a .GIF signature.

 

If ur on a tablet, I just do copy and paste

 

Just use it as if it was an image.