Assault Blog has an excellent tutorial on how to create repeating seamless background images.
You can use textures lots of ways by layering them on top of your images. Here’s a look at how to make your own.
Create the Texture
I started with a new image 500px wide and 500px high and a resolution of 300. If you plan to print the image remember to change the resolution to at least 300.
Pick 2 colors #f61717 (red) and #f0e30e (yellow), or whatever you want, and used the Gradient Tool to fill the background layer.
Select two brushes and doodle inside until it is completely full. I suggest going into the brush dynamics and making adjustments to shape and color dynamic. You can also create additional layers and fill with different brush strokes or patterns then merge them together.
Offset the image by 250px both vertically and horizontally. Go to Filter, Other, Offset in Photoshop. Set the Horizontal and Vertical offset values to exactly 50% of the image height and width and set the undefined areas to wrap around. Looks a little off still right?

Select the Clone Stamp Tool and choose a sample area in the image. Hold the alt button and a crosshair will appear, click in an area away from the seams preferrably in the lower left or lower right area. After clicking release the alt button and your brush will appear again. Use the clone stamp tool just like a brush and draw over the seams. I think it helps to use a soft brush for the sampling, like an airbrush. Or for textured images use grunge brushes
After you’ve clone stamped out the seams your image should look something like this:

Use the Offset filter again by hitting cntrl+F or go to Filter, Other, Offset and hit ok. The image should re-wrap itself by 250px vertically and horizontally and there should be no seams. If there are, repeat step three and clone stamp out the seams. Or if you don’t like the look repeat until you get a texture you do like. Mine turned out like this:

If you’d like to save this texture for your own use, you may. Just right click and save as.
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