PNG Transparency and Printing Grief

I'm having trouble printing PNG images with (supposedly) transparent backgrounds, and I'm hoping one of this blog's readers can help. Here's one of the images from the Software Carpentry course: [image not available] If I open it with the Windows Photo Printing Wizard (on XP), it shows up as dark blue lines on a faint blue background; when I print it on a black & white printer, it comes out as black lines on a white background. Open it directly in Firefox 1.5.0.1 (also on XP), and it displays as dark blue on white. When I print it from Firefox, though, I get dark gray on near-black---sort of, but not quite, a negative image of what I actually want. If I create a vanilla HTML page:
<html>
<body>
<img src="http://www.third-bit.com/swc2/lec/img/py02/negative_indices.292x83.png" mce_src="http://www.third-bit.com/swc2/lec/img/py02/negative_indices.292x83.png"/>
</body>
</html>
and open that in Firefox, I get the same behavior. Open that page in Internet Explorer 6.0.2900, however, and I get dark blue lines on a faint blue background; printing from IE gives me dark lines on a faint gray background. So, what am I doing wrong?
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