Sudden Quality Loss

Been working on this site for the last three weeks and all was going well when…I preview (as I had done before) and all my JPGs are appearing at medium or low quality.

Now here’s the thing: if I right click while previewing, to pull up the contextual menu, and select “High Quality”, they go back to looking great. WTF?? It’s like I hit some switch that auto publishes images at a lower quality. Is that possible?

Any help much abliged.
gR