You are using wrong syntax.....
Plz try This.... may be will help u
<img runat="server" alt="About Us" border="0" src="<%$ Resources:GlobalText, AboutUs%>"
onmouseover="this.src= '<%$ Resources:GlobalText, AboutUsHover%>';"
onmouseout="this.src='<%$ Resources:GlobalText, AboutUs%>';" />
Thanks