Finder uses the raw file data to generate the "thumbnails" on the fly.
If you are seeing low-res icon previews, select one and Get Info (cmd-I). Select the icon in the inspector window and press the Delete key.
If that improves it, the file had a custom icon which Finder will choose to use over auto-rendering.
If not, I have seen some posts that discuss similar problems, but I haven't followed them.
After solving the problem with the existing thumbnails, I have started to generate the missing by using an earlier version of Photoshop. This shows that the newly generated icons are of low resolution. Deleting finder preferences does not help.
ADOBE blaims Apple for the missing thumbnails. In such cases, in the inspector window there is no icon. If you click in that area, nothing happens. Thus, are both are blaime?
In that Apple doesn't create thumbnails at all, you could "blame" them, but I don't see it that way. If there is no thumbnail attached to the file, the Finder uses the file's contents to create the icon preview. If you use an app that creates thumbnails for you, that app needs to create them at a resolution that is not pixelated.
Back when you had to create the thumbnails, icon (and screen) resolution was much lower. So, the older thumbnails are low-res.
If there is no custom icon when you Get Info, then I don't know why it is rendering poorly, unless the Finder cannot interpret the file data correctly. This is what I meant in my first reply in that you need to search here for similar discussions. I know they exist, but I've never seen that problem, so I have never looked into the threads.
The issue is that all files generated on my computer appear in a finder window in the same way except those generated by Photoshop CS6.
Such files have no thumbnail and no preview. I can open them in other appropriate applications and save. The outcome is a very low resolution image. When I delete this image as you have proposed, a high-res image is created. This happens only with Photoshop CS6. No other application behaves like this. No MACOS since 6.1 did something similar.
I am not after the culprits to blame, but after a good solution.
If it is just happening with CS6, maybe it is creating some sort of "preview" image that is part of the file and that image is being used for the icon preview. I pretty much offered what I know about the icon previews. If there is something else going on, then you might find it in the other threads.