If you look in the
MasterPage you will see that there is a
DelegateControls let you swap out controls for controls defined in features at a web/site/application/farm level. For more details see the linked blog post, it's Targetted at SP2007, but the concept is the same in 2010.
You can also copy this file into your own feature as a starting point.
Simply change the two properties for "
GoImageUrl" and "
GoImageUrlActive" (and optionally the RTL versions of them)