Aseprite - Aseprite is an art program dedicated to the creation of pixel art.
Adobe Photoshop - Adobe Photoshop is a webtop application for editing images and photos online.
Pixel Art to CSS - Draw and animate pixel art, export the results to CSS
GIMP - GIMP is a multiplatform photo manipulation tool.
Trixel - Instagram for triangular pixel art
Affinity Photo - Affinity is the imaging and design suite for creative professionals exclusively for Mac.