You've allowed irfanview to be set as the default viewer for all image types.
if you want to change it open Irfanview go to
Options
Set File Associations and then deselect the ones you don't want to use Irfan for automatically or select clear all.
click on OK and exit program.
there are a few more clicks need to associate your images with a specific program
right click on the image concerned select open with and make sure always use this program for this type of file is ticked/crossed/marked and off you go
