Computer Help and Support

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!