Well see here is my problem though. Your claiming there is a problem with the image not being transparent and it is not a problem with your css stylesheet. The problem though is if the actual image we are using is not transparent then there would be an issue with any stylesheet that has a background color different than that of the image. http://www.omegatron.net/gallery/showphoto.php?photo=5
See my thinking here so I did a test here is a nice black background even darker than yours and there is no image issue here. That would lead me to beleive there is some css property in your specific stylesheet that is causing this issue.
I hope that makes a little sense here. I have not debugged your specific stylesheet but if the image is truly transparent which it seems it is then we need to look into why your style is causing the issue.