Wow, I gotta say this is amazing! I was looking for a tool with exactly these features. As a Linux Mint user I am very grateful as all the other alternatives like GIMP, Pinta, Krita are just too much for simple pixel art. And often overly complicated. Have you considered creating a .deb package and add pixelorama to the official ubuntu packages?
EDIT:
Made a donation just now, you earn it. Keep up the good work!