I just size the image in Gimp2 then use the following:
<html>
<head>
<title>kobo menu info</title>
<style type="image/png">
p {
margin-top: 0.5em; }
</style>
</head>
<body>
<img src="/mnt/onboard/.kobo/kbmenu/txt/poweroff.png">
</body>
</html>
The full screen image dimensions for the AuraHD are 1040x 1387. The use of html makes it so you can display variables and generated text easily as well. For most of us, since it will be a static image on the screen anyway, creating the image in an image editing program makes more sense.
Luck;
Ken
Just adding width="1040", as below:
<img src="/mnt/onboard/.kobo/kbmenu/txt/poweroff.png" width="1040">
should resize the image.