A favicon is the small image that appears next to the website title on the title bar . tab. For example, this is Tumblr’s Favicon:

• A favicon needs to be 16px by 16px. You can make your own in a graphics programme, or find one online - just remember to give credit!
• Upload is to an image host, for example ImageShack. 
• Click Customise Appearance, then Theme. 
• Make sure you are using custom HTML. If not, click on ‘Use Cutom HTML’.
• Search for this code:

<link rel=”shortcut icon” href=”{Favicon}”>

• Replace {Favicon} with the url of your favicon. 
• Press save and you’re done!