There are many stock free image sites on the web where you can purchase such images for reasonable prices. However, there are also many sites that let you download such high quality images free-of-charge. Find the best selection the internet has to offer:
1. StockFreeImages - This site belongs to Dreamstime.com and used to be in their free section. In March of 2012, all of the images were ported over to this current site. The images from this site alone has saved me thousands of dollars as I have literally used hundreds of their images for my niche sites. You will need to register one time in order to download and it has a liberal attribution policy (only required where possible). They currently have more than half a million large high-quality images (yes, you read that correctly, it is more than 570K images) for free download. If there is only one site you can choose, you definitely will not go wrong with StockFreeImages.com
2. stock.xchng - This is one of the most well-known and popular site on the internet that has a large collection of free-to-use stock photos. Currently has almost 400K free images to choose from. Registration is a must to use the photos and attribution does not seem to be required at the moment.
3. FreeDigitalPhotos.net - This site has plenty of images that are pretty original and it a very good site to source for new ones. Registration and attribution are required in order to download and use the images. The images, although smaller than most, are still great alternatives to the other sites.
4. StockPhotosForFree - This is a great site with over 100K free high-quality stock photos. Registration is required and attribution is not required in order to use the photos.
5. RGBStock - RGBStock has 72K free images and seems to be an extensive collection of unique photos. Registration is required but attribution is not.
6. Kozzi - Kozzi is a very popular free image resource. With over 50K images, it provides a very good alternative for photos. It also has an extensive collection of clipart for free download. Registration is required but attribution is not required to use the images.
8. MorgueFile - MorgueFile has a good and extensive collection of photos that is free to use and download. Registration and attribution are not required.
9. FreeRangeStock - This is a site with a smaller collection but higher quality images. Some of their photos are taken by in-house photographers and come in good quality sizes. Registration is required to download but attribution seems to be not required.
10. FreeStockPhotos.biz - FreeStockPhotos.biz carries many different kinds of images. Some are from in-house photographers and others can be from contributions or are licensed to be distributed. Registration is not required and attribution is also not normally required, but may depend on the nature of the image.
11. Gimp-Savvy - This is a good resource of 27 thousand public domain photos from a couple of organizations. They are free to use and does not need registraion nor attribution.
12. PublicDomainPictures.net - This is another great resource for public domain images. The images can be used for almost any purpose, including redistribution. Currently has more than 23K images. No registration and attribution are required.
Update: 13. FreeFoto.com - FreeFoto.com is an additional great website that offers more than 130k images that can be used for free. However, attribution and link back is required.
Bonus Free Image Site
Another great site to download free photos is PhotoXpress. There are many free images to choose from, however, there seems to be no way to find them on their site's search engine. In order to do so, you will have to go to EveryStockPhoto and use their "Advance Search" function. From there, uncheck all the sites available and only leave PhotoXpress checked. Using this method, you will be able to find plenty of images for free. In fact, it may be an even better site than all the others. Registration is required on both sites.
Other Free Image Sites
Paid Image Sites
If these image sites are still not enough for your usage, you may consider going the paid route. One of the best I have used is Dreamstime.com. Some of their images can be downloaded for as low as US$1 per image. Simply go to their advanced search function and choose only level 0 images. These images will cost about US$1 each.
For bulk photos, ShutterStock.com has a great subscription plan that lets you download 25 full-sized images per day for US$249/month. This is a total of 750 images and works out to about 33 cents per image, which I think is an incredible deal for their high-quality photos.
Know of any better image sites? Do leave comments or give your recommendations below.