Before we actually dive into finding images that will work. Let's explore how screens and images work. The short video below does a really nice job explaining pixels and monitors in it's most basic terms.
So that explains how images are broken down and displayed on a screen. As the creator of that video eluded to, there is more to screens than that. While I don't want to dive to deep into the science behind how this works, I do want to get into why sometimes images look blurry. Check out the video below.