Shape layers don't have the ability to use a bitmapped fill. What you need to do is use the shape layer as a matte for the fill pattern.
To make a repeating pattern from your image, bring it into the comp as an image layer. Now apply the CC RepeTile effect (it's an included effect with AE in the Stylize folder).
Increase the numbers in the expand right, expand lef, expand up, and expand down sliders until the pattern fills the space you want it to occupy. You don't have to worry if the original layer is too small, this effect will increase the bounds of the layer beyond its original size.
Make sure the image layer is below your shape layer and set its matte to alpha matte "Shape Layer 1" (or whatever your shape layer is called). If you can't see the matte column hit f4.
If you want the pattern to transform with the shape layer, set it as the parent of the fill layer (or vice versa), so that when you move, scale, or rotate one, the other will follow.