Every picture has a baseline resolution. Think of it like an old-fashioned tv screen. If you go up to the screen and look really close...all you're going to see are little colored dots. You stand back and all those little colored dots produce an image...but up close they're just dots.

Same thing with all digital images. You keep zooming in, and eventually you're just going to be looking at pixels. The higher the resolution the more pixels per square inch...but eventually, you get down to the point where there's nothing smaller to see.