Hi Mike! Welcome to Panorama! (Mike and I chatted by email last night, as he was evaluating Panorama.)
Normally, stacking of objects in Panorama works just like any other graphics layout program, for example Keynote. You can select an object, then use the Object>Arrange>Bring to Front/Send to Back commands to alter the Z-order. This can also be done by right clicking on the object. There isn't a specific documentation for this, but it is mentioned and demonstrated in the Selecting a Completely Hidden Object section of the Selecting Form Objects page.
You might want to start by creating a few colored rectangles and playing with this if you are not familiar with this from other programs.
However, you want to do this with Image Display objects, not rectangles. And unfortunately, there is a bug there. This has been a very frustrating bug because it involves weird behavior of the Apple API Panorama X uses to draw the images. When images are drawn with this API, the always appear with a Z-order above everything else in the window, no matter what Z-order you specify. So it is possible to stack images, but you can't stack something else on top of them, like text. It's probably not much use to you, but here is a link to this bug report in our system:
There is one situation where Panorama X uses a different API to draw the image, and the Z-order stacking does work. I think this might work for your application. If the image alignment is set to Tile, then the Z-order stacking will work correctly (I just double checked to make sure this is true). Of course you don't want your image to tile, but if the Image Display object is exactly the same size as the image being displayed, then only one "tile" will be displayed, so essentially it will be the same as not using the tile option. To learn more about the Tile option, see the Image Display Object page (the Tile option is near the bottom).
By the way, I did verify that when stacking PNG images the transparency works, even translucency (partial transparencty), and I was able to stack both images and text, as long as I used the Tile option. Since you say that you can make all of the images have identical dimensions, I think this will work for you.
I am assuming that by "assemble a new image" you mean that you want to stack multiple images and text on top of each other so that they look like a single image, and possibly print like a single image. Stacking the images as Gary and I have described won't actually create a new image, it will just look like one.