Favicons are the little icons that appear in browser tabs. Wagn will try to make one from your logo, but this may not work with all browsers, and in any case you can offer an image specifically for this purpose.
Here's the *favicon for this Wagn:
Simply create a *favicon card, set its Cardtype to Image, and upload your desired icon. To be compatible with as many web browsers as possible, make sure to use the .ico file format.