Maybe for websites like Instagram that consist primarily of images. For everywhere else you have to amortize the cost of the download over the savings for the number of images in an average browsing session, as browsers segment the cache so you can't assume it will be available locally hot.
Actually I wonder, why in general not more decoders are just put into webassembly and are actually kept 'hot' on demand. Couldn't this also be an extension? Wouldn't that reduce the attack surface? I remember a time when most video and flash was a plugin. People would download the stuff. On the other hand using a public CDN at least would keep the traffic down for the one hosting the website.
Browser makers could easily let resources opt out of cache segmentation and then if everyone agreed on a CDN, or a resource integrity hash of a decoder, the wasm could be hot in the cache and used to extend the browser without Chrome needing to maintain it or worry about extra sandboxing.
They don't do it because they don't want people extending the web platform outside their control.
