Hm, y'know what, that's a great question!
I have not received feedback on this to test if it is or not (I have an animated GIF as a thumbnail & header in each issue). About 70% of my subscribers use Gmail however, so I've been adhering to their design standards at this point for maximum impact and ideal experience for readers. I also try to make sure they're optimized as much as possible without sacrificing framerate or choppiness.