Okay, here is the code so far...
<div class="ui grid" id="logo" style="margin:30px 0px 30px 0px;">
<a href="{home_url}">
<img src="{template_url}/images/logo.png" alt="" width="360px">
</a>
<img src="{template_url}/images/headimg.png" alt="" width="300px" style="float:right;margin-right:30px;">
</div>
The logo.png is 360px x 100px, the headimg.png is 300px x 100px
With this code, the logo.png is shown correctly, the headimg.png is scaled to 272px × 91px and i don't know why.