Not solving the photo sizing problem, but this will work.

You said you have Irfanview; open the pic you wish to use in IrfanView.

First, in Image menu select Resize/Resample and make a note of the image size in pixels. Close that box.

Next go to the Image menu again and select Change Canvas Size.
Select Method 2 - set total canvas dimensions in pixels

Here you need to set the canvas size in proportion to your monitor. If it's a standard size ie 16 : 9, try setting the canvas to 1600 x 900 for example. The trick is the canvas size needs to be bigger than the image size (and you know that from the first step).

Finally Anchor - Center, choose your canvas colour.

Save and set as wallpaper!

Except it's not centered.
Centered but -- has more margin on the left side of the photo?

FIXED it.
Went to PC settings and clicked on 'center'.

But the info you gave helped size it and get it there.

Many thanks.

(Now, if I can do this in the future)

In Win7, the 'Personalization' control panel has a 'fit' option when you use it to select a background image that centers the image.

(Edited to remove: Image>Change canvas size suggestion since canetoad beat me to it.)