Bottom line. If you have a 72dpi image with a 4" x 5" dimensions, and you print it at the same dimensions, you're screwed! I know this but my friend does not. He will think you can get away with using 72dpi image regardless of what size you print them at… LOL! I feel a teaching moment coming on…
I had that very same moment this last weekend, followed immediately by a "Kids these days..." moment. He literally could not see the pixellation. I got out the magnifying glass, and the face of the teenager in question acquired its mind-blown expression.
Now I'm getting multiple low-res-camera-phone pics each day, with some comment like
"it's everywhere, bro! how did i not see this before"