You have to make your layout image beforehand. (A blend, perhaps?) Keep in mind that the edges of this image will be cut off.

After you're done with your image, go to Edit >> Define Pattern. You can delete the pattern after you're done with the layout.

Make a new layer. (This is important!) Use the brushes that you want your image to overlay on this new layer. Make it large! [Screenshot]

Right click, in the Layers window, on the layer you just worked on. Go to Blending Options, and check off Pattern Overlay. Select the image you defined as a pattern beforehand.

My end result: [Screenshot]