I always prefix my images with HttpRuntime.AppDomainAppVirtualPath
Like this;
background="<%=HttpRuntime.AppDomainAppVirtualPath %>/Content/images/aqua.jpg"
Also I think your imgs folder needs to be under the Content folder.
src="/content/imgs/img1.gif
The shortcut to this is actually the "~" (tilde) character, but I prefer to write it out.
src="~/content/imgs/img1.gif"
You could also create a static variable to hold this value,
public static string VP = HttpRuntime.AppDomainAppVirtualPath;
Then on you page do it this way,
background="<%=VP%>/Content/images/aqua.jpg"
This is much easier than writting a helper to return a simple string.