The best way to make a montage of images, using
Irfanview, is to create one using the
Create New (empty) image... option.
You just need to make sure that
Only 1 instance of Irfanview is active is NOT checked, in the options menu (
Options / Properties... / Msc.1)
Using
Image / Create New (empty) image..., set the empty image to the size of your desktop.
Now, open one of the images you want to use, in another instance of Irfanview.
If you want to use the whole image in your montage, select
Edit / Copy.
If you just want to use a part of the image, use the Selection Tool to select the part you want, then
Edit / Copy.
(
More information on how to use the Selection Tool, can be found in this tutorial, under Step 3.)
Once you have it copied, go to the blank image you created, select an area for the image using the Selection Tool, then
Edit / Paste.
This will add the image you copied to your empty image.
Click outside of the selection area, to remove the Selection Tool.
Just keep repeating the process, until your montage is complete, then save the completed image.
Here's one I did of the grandkids, for the wife's desktop: