PNG Transparency plugin troubles

Hi,

Some PNG images are actually not displaying. At first I thought it was a positionning problem, but nothing seemed to be wrong on that side. Then I thought that the picture was not loading. So I set the width and height of the element to fit with the size of the image and added a 3px solid red border. When I reloaded the page, the picture appeared with a small 6*6 red square on the top left corner of the image, as if there was no picture inside the border.

What could cause that trouble and how to fix this?

Thanks a lot in advance,

Blaise Bernier