Wikipedia says: "data file or structure representing a generally rectangular grid of pixels, or points of color, on a computer monitor, paper, or other display device."

Windows/OSX Equivalents:
Adobe Photoshop
Corel Paint Shop Pro
Macromedia Fireworks
Corel Painter - try Krita or Gogh for Natural media painting or look below for ArtRage

NOTE:
Adobe Photoshop 7 and CS2 work under Wine or Crossover Office. As does Fireworks.
Also ArtRage is reported to work well under Wine. Look here.

If you want completely open-source and free, but don't know which one to choose, then here is a comparison between Krita and The Gimp.