Icon and cursor authors should consider the hardware and software configurations of their target audience. If your audience is not limited to specific hardware and software configurations, you should include multiple image formats. Doing so will guarantee that the images appear as you intended.
48 x 48
32 x 32
16 x 16
24-bit with 8-bit alpha (Windows XP)
8-bit (256 colors)
4-bit (16 colors)