The screen size of mobile and desktop are different, then different logos should be used. This feature has been updated to T3v2 framework and recently templates (Eg: http://templates.joomlart.com/ja_business/). However, the Iphone logo are missing in some templates like community_plus. Those templates will be updated soon.
At the mean time, you can add following css to enable logo for Iphone layout:
Open or create file templates//css/iphone/template.css, add this style:
h1.logo a { background-image: url(../../images/iphone-logo.png); }
and update your iphone-logo.png in template images folder.