You would need to set the back ground (box) as a transparency in your paint
program

Denyse,

Is the client see the black box when the presentation is printed or
'on-screen'

if it's on-screen double check there really isn't a black box around the PNG
graphic in another package other than powerpoint.

if there is no black border, re-insert it into powerpoint

maybe someone added a black line (line) to the graphic in powerpoint?

Cheers
TAJ Simmons
microsoft powerpoint mvp

awesome graphics for powerpoint
http://www.PresentationPictures.com
free powerpoint pictures, tutorials, hints, tips and more...

Denyse,

Also, note that this is sometimes a PC to Mac issue.

As well as what the others said we had a very similar problem (client had XP
though) Turned out that updating to SP3 solved the problem