It's actually spelt 'oekaki' by the way.
GIMP, SAI, Photoshop, Imageready, Corel painter... blah blah... Most of these aren't free, but you can download the trials. I'd advise you don't use online graphics things. They just suck.
Well, here's a list of some raster graphics software:
http://en.wikipedia.org/wiki/Category:Raster_graphics_editors