Conflict with single and double quotes

It’s sometimes easier to work with sprintf():

 'Default'  => sprintf( 
     '<img src="https://wordpress.stackexchange.com/questions/213508/%s/admin/images/default.png" 
           width="120" 
           height="80" 
           alt="%s" 
       />',
      get_template_directory(),
      esc_attr__( 'Default', 'my_theme' )
); 

where we use the esc_attr__() to return a translated value that’s safe to be used in an attribute, but I’m not sure what your THEMEROOT contains, so I just replaced it with get_template_directory().