That way, when the browser tiles an image in the background, there is no visible line marking the edge of the image. Images that are not seamless make very ugly backgrounds. A picture of your kitten, or of you family is a poor choice for a background image. If you want to showcase these images, use them as inline images. They will have more contextual meaning that way.
More abstract backgrounds are better choices.
Next, make sure that the text you place over an image is readable. There is no point in putting text on a page if it is going to be illegible because of the background images. The best background images do not have lots of contrasting colors. The more colors a background image has, the less likely it is that you will be able to come up with a working color scheme. What color you use and how opace you make the new layer is once again dependent on the image you're using.