I agree with Tom, but just in case you're using PowerPoint 2007, right-click
the background and choose Format Background. In the Format Background dialog
box, click the Picture category. There you can increase the brightness.
And, in case you're not aware of the standard image tools, when you insert
the image on the master (as opposed to making it the background), select the
picture, which displays the Picture toolbar. Then use the More Brightness
button repeatedly until you get the results you want.
Author of How to Do Everything with PowerPoint 2007