Great, we've ruled out one possible cause.
Looking at the element in chrome, there is a computed style for the background of the button:
This is beyond my skillset to fix but I can offer two (hopefully) useful points:
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Marketo's sample images are larger than the button
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There is a CSS attribute called hover that changes the background when you hover over the image. I can't find the specific line thats causing this, but its likely the source of the grey background
This should be enough information, at least, to take to your developer, or perhaps someone else in the community can offer a specific solution.