Dynamically Insert Image Into Stylesheet

Typically with themes that support custom styles/images, you define everything you can in your stylesheet, and then add the dynamic styles inline:

.header {
    background-position: 50% 50%;
    background-repeat: no-repeat:
    background-size: whatever;
}

And in your templates:

<div class="header"<?php if ( $image ) printf( ' style="background-image: url(%s);"', $image ) ?>>
   Title
</div>