I prefer to make resources for my learners using photos rather than clipart, however these can be hard to come by for free or low cost. I did happen to stumble across the site freeimages.com today while looking for a new background image for this blog. I was actually pleasantly surprised to see how many free images there were.
First sign up and you will receive a confirmation email. Once registration is finished, you can search for the subject of photograph you are looking for.
You will then be taken to the search results page. You can choose to look through them all or click the Free Files tab. There's also Advance Settings on the left to filter results: Order, By Photographer, Date Uploaded etc.
Once you have clicked the image you will see all the details and the photographer. You can then choose the size to download.