To place an image within a heart (or any other shape)
1. Open image and rename to make a regular layer
2. Add new layer and move to bottom of layer stack, fill with background color of choice
3. Select the Shape tool. In the option bar, select PATHS and desired shape
4. Use mouse to make the shape desired size, shape and position.
5. Switch to the Paths palette and click on 3rd icon to Load path as a selection.
6. Return to the Layers palette, make the photo layer active and click on the Layer Mask Icon
The heart shape will automatically fill in the layer mask.
The photo will appear within the heart shape. Invert the mask and the photo will frame a white heart.
7. Add Layer styles if desired or continue with other retouching.

forum discussion: http://forums.dpreview.com/forums/read.asp?forum=1006&message=24277056