OK, you need to make the white parts of the top image transparent. Not all graphic file formats support transparency and you won't be able to do it with Word unless it was transparent before you put it into the document.

So you need to open that image in a bitmap editor like photoshop or Paint.Net. In those tools you can delete the white portions and export the image as a PNG with transparency. Then you import that PNG into Word and apply a drop shadow effect.

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Andrew Lockton
Chrysalis Design, Melbourne Australia