To allow editing the images in external editor you need to set the path to editor first. To do that in General tab.
select command and press inAlternatively right click on ant white space on the Frame and select
option in context menu.In the dialog window enter paths to external editors. Path to MSPaint is predefined (since MSPaint is included in Windows standard programs).
If you plan to use IrfanView filters and effects enter the path to this program on your computer.