IrfanView is mostly an image viewer. However, it has the ability to use other editors that you have installed on your computer to modify the current picture that is viewed. The screen that you talk about is the screen for this kind of job. IrfanView lets you use 2 editors. To change / modify the available options, you need to open IrfanView and go under Options > Properties / Settings (or press the P key) > Miscellaneous. From there you can choose what programs to use for screen 1 and screen 2. To open the current file in screen 1 (using the first tool selected from the above menu) just press Shift + 1 key.