I have found numerous cases that people using the formal white theme, after the site upgraded to Moodle 2.6, the site logo is gone.
before M2.6, you had to put in a link to your logo. Hoverever, in M2.6 you have to use filepicker to add the logo.
According to formal white Custom Logo notes "Change the logo for this theme by entering the full or relative URL to an image you wish to use (i.e. http://www.yoursite.tld/mylogo.png or ../path/to/your/logo.png)" There is no way to entering the full or relative URL to an image that I want to use