If you’re using GIMP and Irfanview (in Windows), set up GIMP as your external editor in Irfanview’s Options, under ‘Misc 2’ tab by filling the “Set external viewer/editor” field with “C:\Program Files\GIMP-2.0\bin\gimp-win-remote.exe gimp-2.2.exe” (not including the quotation marks, i.e. only fill in what is inside them). Change the version numbers if you’re using something other than GIMP 2.0 and Gimp-Win-Remote 2.2.
Now you can go directly from viewing an image in Irfanview to editing it in GIMP by selecting ‘Open with external editor’ from the File menu.