Header image looking blurry (sidebar images too)

https://vrpsych.ict.usc.edu/

Above is the link to the site. When I open up the actual header image in Photoshop, I notice it is slightly smaller (1/3 of a cm) than the actual header being displayed on the site, therefore making it blurry in order to accomodate the bigger size. What’s weirder is Photoshop reads the image at 850px by 140px, and so does the site under where to place the header reading, “Exactly 850 x 140 pixels”.

My header is looking slightly blurry no matter what I’ve tried (gone into my css’s header’s height/width sizes; gone into function php and attempted to make the image header area smaller—this only cuts out the frame but keeps the header image the same; uploaded an entirely new header and reverted back to the old). Nothing is working.

I’ve tried gif, bmp, jpg, and still to no avail. This issue spreads to my sidebar images as well EXCEPT for images inside my content area (as you can see in the Motor Rehab and Mental Health pages.

Any help would be greatly appreciated.