New icon for the "Histogram" button.

Own artwork created with GIMP.
Add icon for "Brightness & Contrast" button.

Icon made by Freepik from Flaticon.com, CC-license.
New icon for the "Calculator" button.

Created by "Wallec", license "CC Attribution-Noncommercial-No Derivate 3.0"
http://www.iconarchive.com/artist/wwalczyszyn.html