This is an example that I used in KSM 6 on a Touch (600x800).
Code:
<html>
<body>
<img src="/mnt/onboard/.kobo/kbmenu/images/test01.png" width="566">
</body>
</html>
I did not bother to cut the image, but its ratio was already approximately fitting (actually it was 540x720).
You can find some other examples with images in .adds\kbmenu_user\mynotes\pofftest of KSM 08.