http://www.designertoday.com/tabinde...opDefault.aspx
Apply Image is one of those Photoshop commands whose dialog is NOT intuitive, but whose functionality (combining images) can be very useful once you get the hang of it.
Important: Images must be the same physical size and resolution. One way to achive this requirement is to apply the the crop tool with specific dimension and resolution specified to the subject images.
Check out Photoshop HELP, too. The help topic will help connect a few dots.
~Danny~