Open the images you will be using for your fill. Click on the small triangle in the background/fill style, select pattern.

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With your mouse move over the pattern, you will see the colour dropper, click the pattern to get the pop up menu. Here you can change the size, angle and load a new fill. Do this by clicking on the arrow for the drop down box.

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Doing this opens up the pattern palette. Here you will have all your preset patterns, scroll to the top and you will see all the images you have open in PSP, to select simply click the pattern you need.