Duplicate your starting image and close the original for safekeeping. Working on a copy prevents the chance of accidentally overwriting your original file.

1. Rename the background layer (Layer 0) so that it is unlocked and layers may be rearranged.
2. Create new layer (Layer 1) and
3. Move it below Layer 0.
4. With the bottom layer 1 active,
Edit-Canvas Size and increase size enough for area of desired drop shadow
5. Fill Bottom layer (Layer 1) with color of choice. Add background texture or pattern if desired.
6. Make the image layer (Layer 0) active and click on the Layer Style icon at bottom of palette.
Select the drop shadow style and make adjustments for desired effect.

Try experimenting with some of the other blending styles to add additional effects.