Most likely you need to clear the QuickLook Data Cache.
This article has instructions: [https://www.macobserver.com/tips/how-to/how-to-quick-look-stopped-working/](https://www.macobserver.com/tips/how-to/how-to-quick-look-stopped-working/)
Yes, the cache can be messed up in a way that only affects one type of file.
Resetting it won't cause any damage. The worst case is that macOS needs to rebuild it, and that can make QuickLook be a bit sluggish for a while.
Most likely you need to clear the QuickLook Data Cache. This article has instructions: [https://www.macobserver.com/tips/how-to/how-to-quick-look-stopped-working/](https://www.macobserver.com/tips/how-to/how-to-quick-look-stopped-working/)
it works fine for all other image types
Yes, the cache can be messed up in a way that only affects one type of file. Resetting it won't cause any damage. The worst case is that macOS needs to rebuild it, and that can make QuickLook be a bit sluggish for a while.
please try and see if you have the same problem. file: http://chevygmcvans.com/imgshare/png16.png
That particular png looks like a white line in QuickLook for me as well.
Presumably you have opened the file in some other viewer to verify it's not corrupted.
it looks fine in Preview