Thx mmbenya, the problem with seashore is the lack of speed. Is a good starting but actually u can’t paint a 1500x1000 at 72 dpi image. I think is a good starting and I wish that in a near future, seashore will run as fast as gimp in linux, but from now…
A really good software I’d like to use in mac is opencanvas www.portalgraphics.net (japanese) www.portalgraphics.net/en (english), but they said that this program will be windows only
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Painter and Photoshop are really good programs, but too expensive for people who wants to get started with digital painting. The “reduced” versions, essentials and elements, respectively, aren’t “complete” enought to work fine with (no brush editing, for example).
…etc etc 
cheers!