When you are working with master pages in SharePoint, there are times that you will put in your own custom images. This is fine, except it can often be tricky to get the src attribute right if your master page is going to be used in a site collection that is not at the root of the web site (for example /sites/mynewsite).
To get the image URL right, use this code:
The important thing to note here is the runat=””server”” attribute, without it the tag will just spit out the code as it is without inserting the correct URL. So by doing this your master page will become mobile enough to be run from any site collection, no matter where it resides.