or make sure your transparent images are gifs or pngs. Both of those file formats allow for transparency.
What he will need is variable transparency (also called alpha transparency), which png (not gif) supports but IE doesn't.
IE is your problem. You can't have variable transparency in IE and this severely limits graphic design.
Variable transparency support is coming in IE and present in all other browsers. But if you want it now you need to use some "hacks" and workarounds.
For a great write-up on alpha transparency along with some js workarounds see here: