For people who don't have Adobe Photoshop, here's a free and cross-platform alternative:
Using ImageMagick, it can be done in command line
convert lQchR.png -auto-level -white-threshold 78% fish.png
This may require some fine tuning of the white threshold value, but it's quicker than GUI especially when you have multiple pictures:
convert *.png -auto-level -white-threshold 78% x-%04d.png