What is the problem with an ImageList!?

I create an ImageList and put some images into the list, which in some areas are transparent and has some round edges; e.g. a round button possibly with shadows. When I use the image as a background image or an image for a button then I get a strange line on the border between the transparent area and the solid area of the image. However, it only occurs at the places which is not a vertical or horizontal line. Strange right!?

I have searched for a solution or known problems, but I can't find anything. Do you know about this problem, or what do I do wrong? If I retrieve the images and doesn't put them into the ImageList structure the problem isn't there!

Thanks, Jacob.

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