well, i have the same problem in a collab that i made in another forum, and i can tell that the problems IS the copy/paste method with png files, when it is used with IE.
In fact, IE tends to dark any png image, and when you copy it to the clipboard, it goes already with the dark palette.
The problem can be solved when people do the "save as" in his HD, and then open the png file with your favorite drawing software, do the editing, and then save again as png.
Hope this help
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