Bitmap to Icon Wizard lets you create high-quality, multi-size icons from bitmap files. It supports generation of icons with transparency, including 32-bit, alpha-channel transparency. Bitmap2icon takes a large bitmap (which must have a clearly defined background colour if the icon is to have transparent areas).

Bitmap2icon supports four colour depths (32-bit with alpha channel, 24-bit, 8-bit, and 4-bit) and five icon sizes (256x256, 48x48, 32x32, 24x24, and 16x16). The large bitmap you supply is automatically resampled to produce these device images. If you have enabled transparency, it generates the standard/alpha mask in this resizing process. You can choose which colour depths and sizes to use in your icon file.