I've mentioned before that you can find great images on Flickr and stock.xchng. Just make sure you check whether the photographer is giving you permission to use the image. Sometimes you just need to send a message asking for permission - I find that most happily give it.
(Creative Commons licenses, used frequently on Flickr, are the easiest way to make sure you have permission to use an image.)
Here's a quick way to search for images on Flickr. With Compfight, you put in a search term and the screen fills with thumbnail images to choose from. It's quite fast.
Here's a screen-shot after a search for "Toronto".
Kudos to Mike Sansone at ConverStations for discovering this for me.