Images are inline elements, just put it in the same container. To do what you said I'd generally do something like:
<div id="header">
<img src="img1.jpg" alt="img1" style="width: 200px;height: 200px;" />
<img src="img2.jpg" alt="img2" style="width: 200px;height: 200px;" />
</div>
Alternatively like auntnini said you can float the images.
Regards,
LC.