Hi LaurentC, yes it's possible, all you need to do is:

1. create an alpha channel around your logo (or draw/create a path - both methods work)
2. save the file as a tiff (include alpha channel)
3. open PowerPoint and "insert picture from file"
4. locate your "tiff" and press okay.

let me know if you have any trouble,
:)
retrocity

ps. sorry for the BIG screencap...