Free Images for Commercial Use
As you already realized, it is easier to find free software, script or plugin than free image you need for commercial use. It is as it is and I am not going to change that, instead I will give you couple of links to sites and resources with absolutely free images for any kind of use including commercial.
Keep in mind that this is not most comprehensive or ultimate list, this is time saving list without any surprise in the task to find free images for your commercial project under low budget. Feel free to leave comments with link to the resources that are not listed.
Morguefile is my first pick. It may took some time until you find god image, but at least you know there won't be surprise like free for personal use only or you reached download limit.
picjumbo is less comprehensive resource but resource with pretty high quality in comparison with other sites. Owner is posting one, at least good quality free image per day. It is worth to subscribe to feed or email newsletter what is offered along with premium membership which is not requirement to download images.
raumrot is a free resource with high quality free images for personal and commercial use with option to download entire collections.
10ravens is pretty good resource with more than 14.000 free images and textures. The only requirement is free account plus restriction to 20MB of files per day.
Mayang's Free Texture Library allows you to download up to 20 files per day. There are currently more than 4350 textures in library.
Wikipedia Public domain image resources is most comprehensive resource with lot of linked sites which offers free images for commercial use. Check it out and bookmark every site you find that can be useful for your projects.