pictures - How do I adjust transparency of watermark in Microsoft Word 2010?

I added a watermark to my page from

Page Layout > Watermark > Custom Watermark... > Picture Watermark

However, the resulting watermark was dim and fainted like it is transparent. I want it to look like the original image; as if I inserted it from Insert > Picture.

How do I adjust transparency of watermark?

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Versions:
Office: Microsoft Office Professional Plus 2010
Word: 14.0.6112.5000 (64-bit)