It's pretty easy, to be honest, but hardly anybody knows the trick,
so I'm posting a quick How to =)
Step 1 - Get a Binary/Hex Editor. I use BinEdit, because it's free and simple.
So I'll go with that one in this How to.
Step 2 - Pick a GIF or make one. Better make sure it's around 100x100px, so it doesn't get screwed.
Step 3 - Open up Binedit and open your Gif.
Step 4 - Select all code (just use [strg]+[A]) and go to "Special Edit -> Hex Editor".
Step 5 - In the popup you need to scroll down. The last number should be "3B", didn't encounter a case where it wasn't.
Change that "3B" to "3C 2C", yes, that adds one number to the file.
Step 6 - Save the GIF, wether you just Save it or Save as another file is up to you, if you're unsure better don't overwrite the old one.
But better don't use Eyecancerish GIFs that are flashing around all the time, for the sake of your followers ;)