To allow editing the images in external editor you need to set the path to editor first. To do that in select command and press in General tab.

Alternatively 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.