Gidday ,
Try imageJ, create a stack of all the images.
use the drop menu selection image>>stacks>>Zproject.
Select either sum, average, medium, or maximum.
Adjust the contrast and brightness
drop menu selection image>>adjust>>bright-contrast.
Should be ok for Lum iimages, a bit tricky for RGB sets.
fin imageJ here,
http://imagej.nih.gov/ij/index.html
kind regards, Alan