HOW TO DO IT
<link href='IMAGEURL' rel='apple-touch-icon-precomposed'/>
If you use a blogger mobile template then you will need to locate this code: <b:if cond='data:blog.isMobile'> (press CTRL+F to open the search box) and paste the same code as before directly below this also! If you cannot find this code in your html, then head back to the top and directly after where you pasted your first code, paste the following code (don't forget to add your image URL again.)
<b:if cond='data:blog.isMobile'><link href='IMAGEURL' rel='apple-touch-icon-precomposed'/></b:if>
HOW TO USE YOUR ICON
Head to your blog on your phone and then press the share button at the bottom. From here you can choose to add the site as a bookmark and / or add to your home screen. From here you can change the name of the bookmark or short cut and then press Save / Add.