I'm wondering which is the best solution for an background image to fit every screen, every browser. Also I've noticed that the majority of techniques crop the image a little bit. I know there are a lot of techniques to do this but I want to know which is the best in your opinion. Thanks!
This post from CSS-tricks.com covers different techniques for full screen background images and includes a fall-back for old versions of IE (I haven't tested the IE fix).
When I don't need to worry about supporting old browsers, I generally use the CSS3 method:
html {
background: url(http://lorempixum.com/1440/900/sports)
no-repeat center center fixed;
-webkit-background-size: cover;
-moz-background-size: cover;
-o-background-size: cover;
background-size: cover;
}