ICO: Icon Image File

Overview

ICO (.ico) files are image files containing one or more small icons used by Windows applications, websites, and desktop shortcuts. ICO files can store multiple sizes and color depths of the same icon, allowing the operating system to display the most appropriate version.

ICO files are essential for Windows branding - they appear as application icons, file type icons, favicon.ico for websites, and shortcut icons. Modern ICO files can contain high-resolution icons up to 256×256 pixels.

History

Development History

• 1985: ICO format introduced in Windows 1.0
• 1995: Windows 95 adds 32×32 icon support
• 2001: Windows XP adds alpha transparency
• 2007: Windows Vista supports 256×256 icons
• Present: Still standard Windows icon format

Key Features

• Multiple icon sizes in one file
• Various color depths (16, 256, 24-bit, 32-bit)
• Alpha channel transparency support
• Common sizes: 16×16, 32×32, 48×48, 256×256
• BMP or PNG compressed images inside
• Favicon support for websites
• Application and file type icons
• System tray icons

Common Uses

• Application icons: Windows program icons
• Favicons: Website browser tab icons
• File type icons: Custom file associations
• Shortcuts: Desktop and Start menu icons
• System tray: Notification area icons
• Folders: Custom folder icons
• Browser bookmarks: Bookmark icons

Advantages

• Multiple sizes in one file
• Universal Windows support
• Transparency support
• Small file sizes
• Web browser compatibility
• Easy to create
• Well-documented format

Limitations

• Windows-centric format
• Limited to small icons
• Less flexible than SVG
• Multiple files better than multi-size ICO
• Poor compression
• Modern alternatives exist (PNG)

Technical Information

ICO files store bitmap images with optional compression and support for multiple resolutions.

Technical Specifications