There is no display:run-in;. You can use display:block and position: absolute on the image, plus parameters for its position (top/bottom/left/right, also using percentage values), maybe transform (for centering), and size to position it wherever you want, independently of the parent element.

I’m currently trying to create a header for a website. My problem is, that I try to scale a img inside my div, but the div element resizes with it.

This is my header

I wish to keep the original size of the div (the green part), while being able to resize my cake img

Here is my current css for the header:

.container-header {
 position:inherit;
 vertical-align: top;
 background: green;
 width: 100%;
 margin-bottom: 300px;

}

.container-header img {
display:run-in;
height: 200px;
}

.container-header a {
display: inline-block;
background: red;
width: 100px;
margin-left: 25px;
margin-right: 25px;
}

.header-logo {
position: relative;
top: 100px;
}

Edit:
Here is my html:

<div class="container-header">

<a href="index.html" style="left: 25%"><p>Koti</p></a>
<a href="leipomot.html"><p>Leipomot</p></a>

<img class="header-logo" src="IMGlogo_placeholder.png" alt="" height="30px">

<a href="index.html"><p>Tuotteet</p></a>
<a href="yhteystiedot.html"><p>Yhteystiedot</p></a>

<a href="PHP/shoppingcart.php"><div class="shoppingcart">
<i class="fa fa-shopping-cart"></i>
</div></a>
</div>