Hi James,
First, in SPS, build a test template containing your background image and the overlaid second image. This will give you an idea of the syntax you need.
I set up a rough example:
http://s7g1.scene7.com/is/image/TechSupportEU/testproduct_36005?$instantTemplate$&wid=504
Note the yellow button overlaid onto the red image
Another example using a different base image:
http://s7g1.scene7.com/is/image/TechSupportEU/Backpack_B?$instantTemplate$&wid=504
In the Image Modifiers section of the preset I added:
&layer=1&src=is{TechSupportEU/yellow_b&size=131,42&pos=67,-174}
Note that this approach can only overlay subsequent layers on top of the image called in the url.
For your scenario, it would be better to set up a template and use parameters to swap the images in and out.
In this example i created bgimage and fgimage
http://s7g1.scene7.com/is/image/TechSupportEU/instanttemplatetest?$bgimage=TechSupportEU%2FBackpack_...
Doing it this way, I can leave the bgimage (first layer) alone and just set the foreground image:
http://s7g1.scene7.com/is/image/TechSupportEU/instanttemplatetest?$fgimage=TechSupportEU%2Fyellow_b
Mark