just jpeg icons no thumpnails

Single-click the icon and press option+command+i to Show the Inspector panel for it. Look for the Open With section. Is the default application set to Preview? If not, do so and then click through the Change All… button. Can you now see the images in the Finder, and do the appear when you select the image and press the space bar (Quick Look)?

Ok so it sounds like a corrupted preference file.

In Finder click on Go in the menubar, then select Go To Folder, enter this text

~/Library/Preferences/com.apple.finder.plist

press Go

The file will be highlighted in a new Finder window, move it to the Trash.

Restart your Mac, a new file will be generated.

Once the Mac has booted up make sure Show icon preview has been checked.

Do you see the preview now.

Click on your Desktop, select View from the menubar click on Show View Options and make sure the Show icon preview is checked.

Are you running the Catalina beta by any chance?