You can use Path.Combine with the "base" path, then GetFullPath on the results.
string absPathContainingHrefs = GetAbsolutePath(); // Get the "base" path
string fullPath = Path.Combine(absPathContainingHrefs, @"..\..\images\image.jpg");
fullPath = Path.GetFullPath(fullPath); // Will turn the above into a proper abs path