Piskel, free online sprite editor.
Piskel is not available for iPad but there are some alternatives with similar functionality. The most popular iPad alternative is Pixen. It's not free, so if you're looking for a free alternative, you could try Dotpict or Pixel Studio for pixel art. If that doesn't suit you, our users have ranked 26 alternatives to Piskel and five of them are available for iPad so hopefully you can find a suitable replacement. Other interesting iPad alternatives to Piskel are Dottable (Freemium) and PixelMaker (Paid).