My image previews are missing from finder???

I'm not sure what category this would be best in, so I'm putting it here.

All images on my iMac won't show up as a preview when they are either on my desktop or in finder, I'm not sure what this is...
It just started happening earlier today. I was installing the Nikon software suite because I was having trouble uploading NEF files from my camera. I'm not sure why this would change anything though...?

Please any idea as to how to change this back, is very appreciated!