ICO (Icon)

The ICO file is an image format that started life on Microsoft Windows 1.0 and was used exclusively for storing image icons used by the operating system. Back then, ICO files were limited in size to 32x32 pixels with only two colors. With subsequent versions of Windows, the format has been amended and upgraded to support larger images with more colors.

Although the initial ICO format was not compressed due to their already small sizes (and limited computing power to decompress them in real-time), as the size of image icons has increased over the years, Microsoft recommends that for larger icon files, PNG compression be used for storing the raster data.