WordPress refers to the blog picture as a blavatar, which is a
fusion of the words blog and avatar. The blavatar is used to identify your blog in a variety of places. It’s used as your blog’s favicon, with your blog pingbacks, and as an icon when your blog is added as a shortcut favorite on devices like Apple’s iPhone.A favicon is an image used to brand or visually identify a website, blog, or online property. It appears at the beginning of the corresponding site’s URL in web browser navigation bars and in browser tabs (for example, if the site is bookmarked locally in your browser).
Ada found a website offer favicon maker service for free,see if it work for you:http://www.favicon.co.uk/.