When I work with animated gif, what I usually do to have a proper fallback is actually by editing the gif itself.
I ask one of our designer to set the first frame of the gif with the image I want for fallback. By setting the duration of that first frame to 0.00second, it doesn't brake your animation.
The result is that every mail client which don't support animated gif display the first frame, and all the others clients display the animation.
Hope it helps!
I often use this to hide elements with inline declaration :
We had the same issue here. Indeed, SP support tell us images expires after 2 months if remember well. We decide to use this workaround used by many :
We create pages in Silverpop that include the images and call these for our emails. That way, they don’t expire.