I don't know how to do that, unfortunately
Have you tried it on a default skin? I seem to recall having briefly tested larger images and found that it works there - this could be an issue with your skin.
This is a customisation that goes beyond the scope of customisability we support, so I can't do much more than point you towards the template where you can tinker with CSS until you get it working for your specific needs.
In this case, we created the display with 16x16 images in mind. That's the image size we supply, and we test it with. Of course we understand that what we code against don't always fit everyone's needs, which is why we put all our HTML code in templates where possible, so people who need a level of customisability beyond drop-in image replacement (of the same size) can customise it themselves.
What this means is that we will provide you with directions to change it, but we can't always tell you "replace code X with code Y and it will work" - it sometimes is not that simple even for us.
If you're not skilled enough in CSS to tinker with it and tweak it yourself, you might want to contact your skin developer and see if they can assist you
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