A favicon (short for Favorite icon) is the small image that is displayed alongside a site’s name or URL in the browser's Address bar, Bookmarks list, History list, and Tabs.
The classic favicon.ico is a 16×16 ICO file, often served in either 16-color or 24bit alpha-transparency format. More recently, favicons have been served as 32×32, which is appropriately scaled down in all major and popular-legacy browsers. Cargo currently supports 16×16, 32×32, 64×64, and 128×128 pixels.
Cargo automatically generates the different sizes in the correct format: if you upload a JPG, PNG or GIF image file that is larger than 128×128 pixels, all four sizes are generated. (Cargo does not scale images up, so for example, if your image is smaller than 128×128 pixels, only the three lower sizes are generated.)
Customizing favicons for different resolutions
If the automatically generated sizes are not visually accurate enough for your purposes, you can create a separate image for each desired size and then combine these into a single .ICO file (the format supports multiple sizes inside individual files). We recommend this tool: http://convertico.org/Multi_Image_to_one_icon
As an example, if you want to make two versions of your favicon: one at 16×16 (classic) and one at 32×32 (Retina), you can create separate designs for both versions, then combine the files into a single .ICO file (using the tool mentioned above). Cargo will not compress, resize or make multiple sizes of .ICO files when you upload them.