Walt, I don't associate it with any files at all. On the file tab up top, click that, and near the bottom of the list, you will see MRUD's, click that, and my documents should appear, you click that and it takes you to all your photos. Find the one you want, click it, click open in IrfanView, and you're ready to do whatever you want. Most everything I do is in the third tab from the left at the top, called "Image". In there it has sesizing, enhance colors, sharpen, etc. Just play around with it for a bit, and you'll get it in no time. If you screw one up, just hit return to original, or in the image tab, hit undo.
When you get the picture the way you want it, click file again, and click save as. It will require you to rename the file, so I usually just add resize to the end of the current file name. and save it. After that, go to the edit tab, second from left, and click on "delete(clear display), and that's it.
If you want to save space on your hard drive, go back into your documents, and delete the original photo that you resized a copy of.