Tux 

This is because of the previewas are JPEG images. JPEG isn’t able to hold an alpha channel. If you open the page that holds the actual image, you’ll see the PNG with alpha transparency.

Because of the fact, that all image pages have the same white background (and i totally hope, that the image pages are NEVER be skinnable!!), it would be great, if in the preview images the pictures are getting a white background.

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Coming from a group home page (?)

Aha, I see! And I totally agree with you. There's no reason to change the alpha channel into black instead of white! Thanks for the member tags.

Well, maybe there is a technical reason. I dont know, how the preview images are created. By using ImageMagick, i’m pretty sure, that there are options :)

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Seen in my account recent activity (?)