The iPad and iPhone version of your site can have their own logos. Here's how you do it:
Note: Tablets and mobile devices come with low res and HD screens (apple calls this Retina) so strictly speaking you could have 2 logos for each device. We've simplified the process by taking your HD (Retina) logo and downsizing it for low rez screens.
1. Create a logo for the iphone (mobile) but make it HD (retina) size by changing the dpi to 144. The width should be 320 points or less. When you name the image use @2x.png (firstname.lastname@example.org).
2. Create a logo for the ipad (tablet) but make it HD (retina) size by changing the dpi to 144. Your menu width for the ipad comes from the menu width you set in Designs >>Tablet Layout>>Menu. When you name the image use @2x.png (email@example.com).
3. In the admin for your site open the Designs panel and go to the Tablet or Mobile Layout. Click LOGO. Press the +ADD button to add your respective logos.