I am currently testing v1.30 of the new firmware. It allows you to tweak all factors related to the webcam if your image is too dark or too bright. It can even do some unimpressive emboss filters to your webcam... if you're crazy enough to do it.

Customizing camera settings

Override camera settings temporarily:

Log in to the debug interface. Then list all tweakable parameters:

/usr/bin/v4l2-ctl -L | less

Then you can directly apply parameters on the fly:

/usr/bin/v4l2-ctl -c auto_exposure_bias=15
/usr/bin/v4l2-ctl -c contrast=0

Override camera settings permanently:

Mount the SD card on your computer, and create a file called camera.txt in /boot and put all parameters you want overridden, e.g:

#Tweak the auto exposure bias
auto_exposure_bias=15
#Tweak the contrast
contrast=0

You can edit camera.txt on-target by remounting /boot read-write:

mount -o remount,rw /boot