ICO is an image format used primarily for favicons (the small icons shown in browser tabs) and Windows application icons. An ICO file can contain multiple image sizes in a single file, allowing the operating system to choose the best size for each context.
A proper favicon ICO file should include: 16x16px (browser tab), 32x32px (taskbar), and 48x48px (desktop shortcut). Some also include 64x64px and 128x128px for high-DPI displays.
Start with a square image (at least 48x48px, ideally 256x256px or larger). Use EzConvert to convert your PNG or JPG to ICO format. The converter automatically generates all standard sizes and packages them into a single ICO file.