I know 2 ios pixel art apps and a mac one
Pixel Board (iPad/iPhone) - FREE: Another pixel art drawing tool, its simple for newbies, but dont has many palette sizes for free https://itunes.apple.com/br/app/pixel-board-8-bit-art/id955426436?mt=8
Sprite creator (iPad/iPhone) - $3.99 Again a pixel art drawing tool but a complex one, has lots of colors, sizes, etc https://itunes.apple.com/br/app/sprite-creator/id1078225966?mt=8
PikoPixel (Mac) -FREE A other pixel art drawing tool but, for mac, it has some cool customizations, such size, palette, etc http://twilightedge.com/mac/pikopixel/
i edited and puted the link