Do a Google image search, an in the drop down box, select large images.
Or, do a search for a product you're interested in- game etc.- and you may find that the website has wallpapers available.
Wallpapers are image files so the risk of spyware or viruses is small- just make sure it is an image you're downloading and not a executable masquerading as one:
http://www.computerhope.com/issues/ch000573.htm If you do a search for 'free wallpapers', you'll find lots of links.
However, these are the sort of site that will contain malware or link to it. I did such a search and checked a few links with LinkScanner. The second threw up a warning:
The page you were checking contains a link to the page hxxp://xxx.softlinkers.org/ which is on a site that is known to contain malicious code. You should not click on this link without appropriate anti-exploit protection on your PC.
If you do visit these sites, make sure your computer is 100% up to date so you're not vulnerable to any exploits that may be thrown at you. (Using avast! Webshield is a good idea too!)
http://secunia.com/software_inspector/
