I think you can try this for calling in from a class
System.Web.HttpContext.Current.Server.MapPath("~/SignatureImages/");
*----------------Sorry I oversight, for static function already answered the question by adrift*
System.Web.Hosting.HostingEnvironment.MapPath("~/SignatureImages/");
Update
I got exception while using System.Web.Hosting.HostingEnvironment.MapPath("~/SignatureImages/");
Ex details : System.ArgumentException: The relative virtual path 'SignatureImages' is not allowed here. at System.Web.VirtualPath.FailIfRelativePath()
Solution (tested in static webmethod)
System.Web.HttpContext.Current.Server.MapPath("~/SignatureImages/"); Worked