Most of the instructions I've found online (this, for example) say all I have to do is select "show icon preview" in the finder view settings. But that doesn't seem to be working. What's the problem here?
This is OSX 10.8.5.
A couple of things to try.
Remove the Finder preferences file. The system will automatically regenerate it.
If this doesn't work, ensure that JPG files are associated with Preview. To do this, right-click on one of them and choose Get Info. Under the Open With section, select Preview, and then click Change All. You may need to relaunch Finder with this method too.
Any chance you have DropBox installed?
If so try this:
Go to /Users/youruser/Library/QuickLook/ and if there is a DropBoxQL.qlgenerator move it to the trash.
Then wait 5-10 seconds, and open a folder where previews weren't working.
DropBox will create a new DropBoxQL.qlgenerator when you login next (either logout/login or on restart) so if you want to prevent DropBox from screwing with your previews again then you'll need to put a dummy file in place.
In terminal run:
sudo touch ~/Library/QuickLook/DropBoxQL.qlgenerator
This will create an empty file owned by root that DropBox can't overwrite (since DropBox is running as your user when you log in).
This hasn't caused any issues I can detect on my Mac Pro (10.11.6) or MacBook Pro (10.12.2) but as always YMMV.
From this link: https://www.cnet.com/news/files-in-finder-not-showing-previews/
From the Finder's "View" menu select "Show View Options". Then select the option to "Show icon preview" and close the options window.
I have OS 10.10.5, and have what sounds like the same problem. I can see the icons again if I go into Finder -> View and then set the icons to the larger size. I can't seem to get the smaller view to work though.
After trying that, I also found that it I go to Finder -> Show View Options, then I can make the font in the finder window slightly larger and then the icons show up correctly. Then I don't have to make everything as big as the first method makes it.