Decorate your Mantel by "leaning" instead of "hanging" frames.
Instead of "hanging" picture frames, "lean" pictures frames. (shown below) You can easily move the pictures around and you don't have to worry about putting hardware into the brick. Layer various different frame sizes and be sure to fill the vertical space between the mantel and ceiling with a taller frame.
This and other great ideas for displaying photos on your mantel can be found at www.BHG.com .