[ excerpt with modifications from developer's site ]

The icoutils are a set of program for extracting and converting
images in Microsoft Windows(R) icon and cursor files. These files
usually have the extension .ico or .cur, but they can also be
embedded in executables and libraries (.dll-files).

The icotool program converts icon and cursor files into a set of
PNG images. (Each icon/cursor file may contain multiple images,
usually of different sizes and with different number of colors.)
Icotool can also create icon/cursor files from PNG images.

WWW: http://www.nongnu.org/icoutils/