Compressed/ Artifacting Facebook Previews... Fine Images on Site

Everything on the site itself looks just fine, but when I link it over to Facebook, the images look terrible suddenly. This is most noticeable with my "general" image. I've tried uploading both jpg and png and changing the resolution with Photoshop, but the result is always the same. Below are screenshots of what the images look like when on Facebook, both are taken from the same window size/ view mode.

This is what the site preview looks like.

This is what the same image looks like directly on facebook.

I'm not worried about the way it crops the image [though that is interesting and I didn't notice until just now] only all that nasty artifacting.

Any suggestions?

Im having the same problem. Its frustrating. Looks like for the "general image" there isnt a Fit or Fill option like there is for regular images. "General images" seem to revert to Fill hence the cropping/zooming and probably why the image breaks down a bit. Also I dont have the option of using a higher rez photo so it looks even worse than yours. 

If anyone knows a workaround or what I might be doing wrong please let me know. Many thanks!