The following technique should work with just about any paint or image editing program. The object that casts a drop shadow should be relatively flat .
Drop shadows work well, e.g., to emphasize a photograph, letters or buttons.
Don't try this on an apple - it will look too unrealistic. A drop shadow typically falls towards the lower right of the object. Examine the original object for signs of a light source. Ideally the light source should be in the upper left. Always duplicate the image and work on the duplicate.