Mixer Mode

Combining two pictures into one can be done in many ways. The most useful methods are provided here:

• Over
  The top picture is drawn on top of the bottom picture. Parts of the bottom picture are hidden.
• Multiply
  This gives the same visual result as two transparent pieces of film placed on top of each other in front of a light source (e.g. two slides on a light box). The resulting image is always darker than the input pictures.
• Screen
  This multiplies the negative of one picture by the other picture. The visual effect is the same as a simple addition (e.g. two images projected on a screen) but the result is better than a simple addition because the highlights don't burn. The resulting image is always brighter than the input pictures.
• Darken/Lighten
  The two images are compared, the darkest or lightest pixel of each input picture is used for the composite image.
• Overlay/Hard Light
  Multiplies or screens the colors, depending on the pixel values of the input picture.
• Difference
  Creates a rich and colorful result using subtraction. The resulting image is a mix of positive and negative images.
• Invert
  Invert the top input picture. This gives a simple result with flat black areas.
• Random
  This uses any of the previous mode in a random way. This is the default value.
  

If you want to know more about mixing/blending modes you can get good information at:
www.digitalartform.com/blendModes.htm