There are several ways to exit the program:
On the main window, press the ESC key on your keyboard. The program will close immediately. (If you’re in any of the menus or dialog boxes, ESC will back out of that window. Press it again to exit.)
From the File Menu, you can press the X key, to close IrfanView. When using the mouse, just click the Exit option to close the viewer.
Click the logo in the upper left corner of the program window to bring up the standard Windows control box and click the Close option or execute a double click on the logo to close IrfanView immediately.
Click the X box in the upper right corner of the program window. This will also close the program.