I tried the 'First Aid/Repair' described above but no luck. Not sure what Terminal is (new to Mac) and didn't find anything on Macosxhints.com.Thought I had already replied to this one. This is the routine.
Try logging out and back in, next try a reboot. Finally try a repair permissions. Go to /Applications/Utilities/Disk Utility; select the boot volume (partition); click on "First Aid" tab; click on "Repair permissions" button.
If that does not work there are a couple of Terminal tricks that might do the trick. Try doing a search for "trash" on Macosxhints.com
geez, i can't even select the file in the Finder, it just gives the same error that it 'can't be found'Select one .png file and do a "Get info.." (command + i) to check if you have full autorisations for the file. Also, booting from an external drive would probably allow you to delete the .png files.