The picture formatting tool bar will accept values that are out of the expected 
range, including null values. 
This is a problem with all boxes - red, green, blue, brightness, contrast, 
gamma, and transparency.
The program appears to limit the values but the user interface does not 
represent the true value that is being displayed in the picture.
For example:
Insert picture into a presentation slide
Select the picture to activate the picture formatting tool bar
-Highlight the text in the contrast box
-type 999 - notice that the third nine changes the display back to 100
-type an additional 9 - it allows this entry to stay
-delete all the text within the contrast box
-click on the brightness box - it allows the null value to stay in the contrast 
box
-If you click on the slide to deactivate the picture formatting tool bar then 
select the picture to reactivate the picture formatting tool bar, the value in 
the contrast box has changed to 1%.