I am unfamiliar with that software, but in most apps you can just have the two images open at the same time (in seperate windows), then drag one image on top of the other. I explain this more clearly in my resizing tutorial (see the tut section on the left) but you would need to use your photo rather than a new blank image.
You can actually do this with Irfanview as well .. but .. you'll have to experiment with it in order to get aspect ratio correct
open an image in Irfanview (1600x1200) .. open a new instance of irfanview with a new image (800x600) .. then go back to the first irfanview image and drag a rectangle where you want your inserted image (*this is the tricky part*) .. go back to Irfanview image #2 and select COPY from the button bar ..
then PASTE into Irfanview Image #1
When dragging your rectangle watch the dimensions of it on the title bar and try to get it the same as your insertion image ..
1) .. you can fine tune the size by moving your mouse pointer to the rectangle lines and getting the "resize pointer" to enlarge the box
2) .. you can reposition the rectangle afterwards via the Right Mouse Button
so now you have, for example, an 800x600 image inside your 1600x1200 original