PowerPoint is not a very sophisticated photo editor, but it can do some very elementary things like increase or decrease a picture's brightness and contrast. The easiest way to access those controls is through the buttons on the Picture toolbar:
More Contrast
Less Contrast
More Brightness
Less Brightness
Each time you click one of these buttons, it changes its setting by 3%. The default setting for each is 50%, so if you click once on More Contrast, for example, the contrast becomes set to 53%.
You can adjust the contrast and brightness more precisely through the Format Picture dialog box (refer to Figure 8-12). Drag the Contrast and Brightness sliders to the right or left to adjust as needed. Contrast and brightness settings are reset to 50% if you click the Reset button in the dialog box.