the problem is that ie5/6 does not support png/transparency. There's no way to make the transparency work. But u can trick it:
use jpg. Instead of 2 layers :background and transparent, you create the image non-transparent(jpg format) but with the background underneath it too... that way, you give the impression of transparency and it's actually a jpg.
if you have any more questions, I am here.
If i helped, please accept my bid.
thank you
Ciprian