[mnutt]

On Mobile Safari at least, you can press and hold the image to share. I imagine there's probably an equivalent gesture on Mobile Chrome. I spent considerable time many years ago trying to figure out how to indicate to users that they could do this. ("press and hold to share", with a progress indicator) Results were mixed.

Fortunately, these days if you wanted to make it more obvious you could hook up a regular Share button to navigator.share() API and pass it your image blob.