Photoshop :: Working With Pictures Already Created.....

I have a web site I would like to use, purchased it from someone who assured me all that I needed to do was "tweek it" to my clients taste. I am now having difficulties trying to put my clients images into the template. The mask which is on the layer is fine, what I would really like to do is to import or whatever it takes to get my client's picture in place of the template picture. I do understand there are layers, and that I need to be on the correct layer to make the change... my problem is that all I want to do is to drop my clients picture onto the layer instead of the template picture. My alternative was to attempt to recreate all the template web site in my own layers... but, that defeats the purpose of buying the template.