I have this code running in an always-on AutoHotkey script, listening for Ctrl + Up/Down/Left/Right combinations. When IrfanView has an open image, pressing the corresponding combination (Ctrl + one of the direction keys) will paste the image in your clipboard to that respective side. It's just a shortcut for Edit > Paste Special (add on side).

Code:
#IfWinActive ahk_class IrfanView

^Left::
	KeyWait LCtrl
	Send {Alt}{E}{T}{L}
Return

^Right::
	KeyWait Ctrl
	Send {Alt}{E}{T}{R}
Return

^Up::
	KeyWait Ctrl
	Send {Alt}{E}{T}{T}
Return

^Down::
	KeyWait Ctrl
	Send {Alt}{E}{T}{B}
Return