Why do images rank so much better than regular pages?

I have a WP site that has been live for about 3 years now, gets about 300 clicks p/d but the majority of those clicks are coming from image traffic from irrelevant keywords that I'm not even trying to rank for. This site used to make a ton of money but then the niche hit some legal turbulence and now it's pretty much worthless.

I've got an image ranking on the first page of google images for a keyword that gets 13.6m global searches a month. This single keyword is generating over 90% of my site's total traffic. The image isn't even mine... I think I took it from WebMD a couple years ago and just resized it.

So why do images rank so well on WP blogs?

Is there a way to make the actual page that the images are on rank higher than the images itself?

Has anyone experimented with different types of on-page linking? like linking the images back to the page URL itself as a permalink?

because people are probably hotlinking those images more than they are linking to your site. Google uses that as a way of working out how important an image is and ranks accordingly.

Because noone bothers with image SEO - like alt, title tags etc. As soon as you do, and have an optimised site, images rocket to the top. I had a site that ranked 1 in images for 'skateboarding' for about two years. Insane amounts of traffic, but my earnings are still the same now without that traffic.