WordPress.com inctoduced a new site logo feature just recently with the goal of standardizing the way themes present logo upload.
When you change a new theme, you lose a logo. Although developers have a their ways of incorporating logo upload, it is not portable across themes.
The new site logo feature lets you display the site’s title, and tagline along with the logo which will appear on any of WordPress.com’s supported themes. Future themes will comes with a site logo feature and WordPress developers are working to support previously released themes.
Read the rest of the news here: http://wptavern.com/wordpress-com-moves-to-standardize-theme-support-for-logos