Image Transparency

I am creating a document in Microsoft Word 2011 (for Mac). I grabbed a pic out of the image gallery and set the transparency to low (making the background not so noticeable). The problem is he has a Windows machine and this transparency setting does not exist in that version. So, when he opens it on his machine, the pic is not transparent at all.

To get around this, I would like to make the pic transparent in a program outside of Word and save it. Then, I will simply bring that new picture into the document (where I do not have to edit it further) and I would think that would then display the same no matter which version of Word it is being opened from. Does anyone have a good program I could use that has this same type of transparency setting? I have PaintBrush, but I am not finding anything like this. Thanks!