Unfortunately not all websites are the same, and some use all kinds of layout tricks that prevent addons like this from realistically working, or even specifically protect their images from copying tools. I feel like you're trying to paint this as a shortcoming or flaw, but that's a little unfair. Webdevs will always find ways to hinder tools like this, on.purpose or coincidentally. There is no addon that works perfectly for all.

Update: yes, of course any visible image is by definition available to download, the problem is with websites doing all kinds of weird-ass shit to satisfy their sometimes arbitrary layout decisions and completely violating web standards in the process, or just using complicated css nonsense that can completely throw tools like this off-track. My addon is able to work through most of those things, but you can't expect 1 generic little piece of code to be able to perfectly circumvent everything a human dev can come up with that makes its life harder. Write an algorithm yourself and submit it in a PR, Mr. Perfect, or be satisfied that it works 99% of the time.