I've been looking for a simple image blocker compatible with Firefox 57+ beta and found this.

Features: Can block all images, has a whitelist, can Import/Export whitelist, placeholder options (None and ALT String), each tab can have a mode independently, take over mode from current tab (new tab will have same mode as current tab), hide context menu extension.

Verdict: It can be a simple image blocker for beginners, and has options for advanced users.

EDIT: After a couple more hours of browsing, I've noticed it doesn't block images on some sites even with block all sites enabled (best example: Wikipedia). To the dev: Any particular reason, or is this a problem on my end? I haven't made any changes in settings.

Also, it blocks social media buttons on some sites, but not all of them (e.g.: I can't see the Like button on Twitter, but Reply and Retweet are visible). This could use a fix.

ALSO also, it doesn't remember my choice, it seems. After I'm done browsing, I unblock images & close Firefox. But when I open it again, the option to block all images remains enabled. I thought this was a great add-on, but now I'm starting to doubt.