I'm not sure if I understand your problem, but it seems you mix up two thins: transparency of the png image and background transparency of the html object "img".
Both are btw. no c5 issues...
To get transparency in your image, you have to refer to your image software (photoshop, paintshop, corel et al...). Setting background transparency to the img-object with css styles is possible, but makes no sense, since the background of the img-object is covered by the image!