I have added that in my latest tutorial.
http://forum.weavertheme.com/discussion/11685/weaver-xtreme-tips-and-tutorial-part-6
if the image does not resize on some browsers, it may need a max-width:100% and or a width:100%.
Try adding it to the image tag
<img src=”MobileHeaderImageUrl” style=”max-width:100%;”/>
or you can add it to the rule
{} #branding %selector% {display:block;} #branding %selector% img {display:block;max-width:100%;}
report back what fixes the issue so I can incorporate in the tutorial