Using your photo editor
There are many ways that a photo editor can be of service in working with your background images. Many of these operations can also be applied to any photo to enhance the photo or for special effects. I most commonly use the freeware photo editor, IrfanView, but most photo editors will have similar functions.
IrfanView has a panoramic function that I use in conjunction with horizontal and vertical flips to build the images to begin with. I take a cut from a photo and flip it horizontally and vertically to form the parts of the image and then stitch them together with the panoramic function of the editor.
I have standardized on a 200 px square format, although some of the first images were of variable size. To arrive at this size I use a single cut of 100 px in four iterations. I present these images in a jpg format. Many of these images could be converted to a gif format. That might save some file size. Your photo editor probably has a way to convert images from one format to another. The gif format does not use as many colors as the jpg format. Jpg is better if there is fine detail in the image. Many of the textured backgrounds would not suffer from the conversion to gif.
The images could also be resized in your photo editor. With images such as the one used on this page resizing would significantly effect the display. There would be less effect with an image like the background of this content area.
With the photo editor any of the images could also be converted to grayscales. The photo editor will also allow you to adjust the colors. There are several controls available. You can usually make an image lighter or darker. You can also change the colors in several ways. IrfanView allows you to swap color spaces and this can change an image dramatically.
If you see an image on the site that is almost right open it in your photo editor. You can probably make it into just the way you want it to be. I have used this at times to alter a photo to make it more pleasing for a particular application.
I have presented the images in both a horizontal and a vertical orientation here on the site. The image is the same, just rotated 90 degrees. If you save an image and then think that it might look better in the other orientation it is a simple matter to rotate it in your photo editor.