Creating a brush to use as a background texture is very easy in Photoshop. There are just a few things you have to do to prepare any image so that it can be turned into a brush for use later. In this example, I'll turn a stock texture image into a brush. Note that I am using Photoshop CS5. Here is my original image:
The first thing to remember is that the largest dimensions that brushes can be is 2500 pixels by 2500 pixels. You must resize or crop your image so that it is 2500 pixels or less in width and height. I did not want to change the scaling of my texture, so I cropped it into a 2500 pixel square:
The second thing that needs to be done is that the image needs to be converted to greyscale using any number of methods available in Photoshop. When Photoshop defines a brush from an image, it will convert pure white into 100% transparent (completely clear) and pure black into 0% transparent (completely opaque). All values in between are pro-rated based on their greyscale value. For this example, I will go to Image - Adjustments - Black and White... Here, you can adjust greyscale values with a little flexibility from the source image's original color:
Here is our greyscale image:
Now that our texture image is ready to be defined as a brush, go to Edit - Define Brush Preset... You will get a dialog, allowing you to name your new brush:
Your new brush should now show up in your Brushes window and be available for use! Let's test it out. Here is a boring old gradient:
Let's select the brush, adjust the size, and with a single mouse-click, we get a nice texture with our background showing through:
That's all there is to making brushes in Adobe Photoshop, go have fun!