Actually, to make wallpaper appear correctly on different resolution monitors, you must adjust the aspect ratio. This is only done through cropping, not what we're referring to as "resizing".
The resolutions 1440 X 900, 1650 X 1080 , and 1920 X 1200 all have the same "aspect ratio". (1.6 to 1.0) Accordingly, any picture in one of these formats could be displayed on any monitor of these resolutions and the proportions will be correct, nothing will be distorted, and the graphics driver will add or subtract lines as necessary to fill the display. However 1920 X 1080 is a different aspect ratio, (1.78 to 1.0), so enter the crop tool, or canvas resizing dialog.
I have 2 different resolutions of WS monitors, 1650 X 1080 and 1920 X 1200, yet I do all my wallpapers in 1920 X1200, and it will look just fine when displayed at either resolution. Also, to avoid excessive "resampling" of large files, I will do wallpapers at exact multiples of the necessary resolutions, IE; 3840 X 2400 or 2780 X 1800, which are 2X and 1.5 times the 1920 X 1200 number.
Anyway, the "filters" to which SNGX is referring to in Irfanview, are related to it's resampling algorithms with respect to resampling an image to increase (or decrease) it's size with respect to DPI (dots per inch) for printing or screen viewing.
Most of that above is a slight oversimplification, but I hope it makes at least some sense.
You do have to get a handle on the correct application of cropping and resampling/resizing to acheive the best results possible with respect to wallpaper display strategies.
Basically, for a 1920 X 1080 monitor you can multiply the horizontal size (in pixels) of any picture file you might want to display by .5625 and the resulting number will be the number of pixels on the vertical axis you must have to make it display without distortion. IE; 1680 X 945 would be a size that would also display correctly.
Is that better?