Imagicon is a freeware application for Windows that allows you to convert multiple image files from BMP, JPG or PNG image formats into any of the following 24-bit icon formats:

Windows or OS/2 Bitmap (BMP)
Independent JPEG Group (JPG)
Portable Network Graphics (PNG)
Windows Icon (ICO)

Here are some of the features available:

-Save icons with the following dimensions in pixels: 16 x 16, 32 x 32, 48 x 48, 64 x 64 and 128 x 128.
-Enable alpha transparency of icons.
-Drag and drop support for multiple images for faster processing.

Download from here