If you are using Excel 2007, I found the answer in Excel 2007 Help under "Change the transparency of one color in a picture". This is direct from Excel Help:
"You can make part of a picture transparent to better show any text that is layered on top of it , to layer pictures on top of each other, or to remove or hide part of a picture for emphasis.
Transparent areas in pictures are the same color as the paper on which they are printed. In an electronic display, such as a Web page, transparent areas are the same color as the background.
You cannot make more than one color in a picture transparent. And because an area of what appears to be a single color (for example, blue sky) might actually be made up of a range of subtle color variations, the color that you select might appear in only a small area. Therefore, it might be difficult to see the transparent effect.
1. Click the picture that you want to create transparent areas in.
2. Under Picture Tools, on the Format tab, in the Adjust group, click Recolor.
3. Click Set Transparent Color, and then click the color in the picture or image that you want to make transparent.
The Set Transparent Color option is available for bitmap (bitmap: A picture made from a series of small dots, much like a piece of graph paper with certain squares filled in to form shapes and lines. When stored as files, bitmaps usually have the extension .bmp.) pictures that are not already transparent, and some clip art (clip art: A single piece of ready-made art, often appearing as a bitmap or a combination of drawn shapes.)"