One of the most usefull features in CHDK is RAW format, and this is the results.
Original JPG, darker.
RAW Format, converted by Irfan View JPG, brighter.
RAW Format, converted by CHDK builtin RAW Developer, much better.
RAW Format mode always save 2 images, RAW and JPG for thumbnail view, the RAW files size is about 9MB for 8MP sensor.