I know that end-users making memes or whatever aren't a demographic that will matter for adoption, but it seems really weird that this is still a problem on Windows and Mac in 2026. I've searched for browser extensions to no avail.
I just tried it out now, using Waterfox on macOS 27. The save image dialog literally already has a Format dropdown. But I'm guessing it's just a weird part of some native dialog, it lets me select between "WebP Image" and "All Files". This kind of design could've been used to allow the user to save it as a different format than what it was served as.
If you have support for multiple file types, you're already using multiple libraries or you're using a library that handles several kinds of file, either way you can and should update that.