I have used Photoshop Elements 6 for these instructions, but this technique should work in older versions as well. If you are using an older version, your palettes may be arranged a little differently than what is shown here.
Open Photoshop Elements in Full Edit Mode.
Open the photo or image you'd like to use as the fill for your text. For simplicity, I am using one of the free patterns offered on this site.
For this effect, we need to convert the background to a layer, because we will be adding a new layer to be the background.
To convert the background to a layer, double click on the background layer in the layer's palette. (Window > Layers if your layers palette is not already open.) Name the layer "Fill Layer" then click OK.
Note: It is not necessary to name the layer, but as you start working more with layers it helps to keep them organized if you add descriptive names.