The difference is barely noticeable in real-world cases, in terms of performance or battery. Decoding images is a small part of loading an entire webpage from the internet. And transferring data isn't free either, so any benefits need to be offset against the larger file size and increased network usage.
When you talk about images over HTTP, you need to consider most web servers and browsers already support zstd compression on the transport, so the potential bandwidth win provided by zstd is already being made use of today.
