I have downloaded images of places I wanted to go anyway to view later or techniques that I wanted to know more about. Have also downloaded images that I wanted my wife to view and usually we discussed them together. I might even have a couple some where on the computer still that have missed being deleted.
A friend of mine downloaded an image from a professional photographer he had a workshop from and liked the image and wanted to use it as his desktop. He then emailed and got a permission for using it as his avatar and a higher resolution image for his desktop. He was happy and the other photographer was pleased with the request. If I wanted to keep the image for any length of time I think I would do what my friend did as well.
I could not think of any instance I would ever pass off some one else's work as my own. Maybe some of my worse images I should try to pass them off as some else's though