Menu and content overlaps image in header - html

Question

I'm trying to add a site banner above the menu on my site using a simple img tag but whenever I do so the menu just overlaps the image. What I want to achieve is the menu to be pushed down by the image above it so it appears right under it.
HTML:
<header>
<img src="site_logo.jpg" alt="">
<img src="banner_small.png" alt="" id="banner">
<nav>
<ul>
<li class="selected">Main page
</li>
<li>Classes
</li>
<li>Game modes
</li>
<li>Contact
</li>
</ul>
</nav>
</header>
My first img is for the site logo and the second one is in the menu. I want to position the first img to be on top of the site and push the menu down
CSS:
header {
height: 50px;
background: #333333;
}
header nav {
display: inline-block;
vertical-align: middle;
}
header nav ul {
margin: 0;
padding: 0;
}
#banner,
header ul li {
display: inline-block;
vertical-align: middle;
}
header nav > ul > li {
box-sizing: border-box;
height: 50px;
padding: 12px;
position: relative;
}

Answer 1

Try to put the images inside div elements:
<div id='x'><img src="site_logo.jpg" alt=""></div>
<div id='y'><img src="banner_small.png" alt="" id="banner"></div>
Then if you want to increse ou decrease the space between elements use the Margin property:
x{margin-bottom:...px;}
y{margin-top:...px; margin-bottom:..px;}