Something weird happened to my Desktop pictures functionality: most of the default desktop pictures - all PNG files (all tiled solids and wide screen photos from the Library>Desktop pictures) appear banded (regular tight vertical bands of subtle light and dark shade of the pic) when I apply them to desktop from Desktop control panel in System preferences. Swatches in Desktop control pane look perfectly OK. I also checked the picture files in Library>Desktop pictures by opening them in Preview and they look intact. It is just when I apply them to desktop they become banded. JPG files seem to work fine and PNG files appear banded.
I ran Disk utility, corrected privileges but it didn't help.
Since logging as a new user fixed the problem I also trashed the desktop and finder .plist from the main user prefs and restarted but the glitch remains in my main account.
Is there anything else I can do to fix this hopefully minor though very annoying problem?
PB17"/OS 10.2.6