Photoshop is a bitmap based application, i.e. if you look very closely you will see the pixels/jaggies. For an photo that isn't always the limitation you might think. Because they aren't regular, with straight edges etc. you can often get away with blowing an image up more than recommended, especially if it is on something to be seen from a distance rather than held in the hand, like a pull up banner or a mural.
I discovered just how much you can get away with when I once designed an A6 postcard for a client. They were so happy they asked the printer to also produce A1 posters from the same file! I could have easily provided A1 artwork but the first I heard of it was when I saw the poster on a wall. Initially horrified I then realised it looked alright from across the street. It even looked okay from 10 feet away. It was only when I got closer than you really would for a poster that I could make out how ropey it actually was.
The thing that really let it down was text that was incorporated into the image and had been set in Photoshop. If, instead of saving the final image as a TIFF copy (or even the native PSD format) for use in InDesign, I had saved the image as a Photoshop PDF then all the text I had set would have stayed as sharp, vector elements… and even up close the text on that poster would have still looked sharp.
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