Modern image servers can be pretty sophisticated, sniffing your browser and serving a more efficient format (like WebP), even if you request something like a GIF.
Sniffing browsers and serving different images to different browsers falls solidly in the "clever" category, and also in the "unnecessary complexity" category. At least in the context of MV. I'm sure there are contexts where it makes more sense. But our bandwidth bills are not extortionate, and png is so widely supported that it's an obvious choice for a simple solution.
An unstated goal here (that I will now state) is to avoid building something so complex that the part-time and sole maintainer (that's me) can no longer maintain it. In other words, I am quite capable of out-clevering myself, and a lifetime spent writing code has taught me the importance of simplicity over complexity.
I'm also acutely aware now -- in ways that I would not have believed or comprehended when I was 30 or 40 -- that the effects of even a small cognitive decline as one ages will eventually halt my ability to comprehend complex code. It hasn't happened yet, but I can feel it creeping up.