Here are 3 of my favorite microstock photography sites that you might want to visit to set up an account:
Free images can be obtained at StockExchange just about any time, and they're not limited to once a week. I use them a lot and have corresponded with many of their photographers. Just make sure you check the requirements that accompany each image because some photographers require you to notify them if you use their image. That's a small price to pay for a quality image.
I use a lot of images for this blog, my book covers, and promotional materials so I'm always shopping for good photographs. In fact, I have a regular membership to BigStockPhoto wherein I can download 5 images per day with no restrictions on size. This is a great bargain.
Free is definitely a good thing.