I happen to agree with the idea of grounding morality in God, I just think that what most theists think that God is, is wrong.
Religion is (or at least was, before the advent of world travel and the internet) connected to culture. For example, Christianity arose from Judaism, a religion practiced by a geographically circumscribed race. It is in Judaism that the idea of monotheism, as most widely practiced today, arose (though Egypt’s Akhenaten attempted to turn Egypt’s religion into a monotheism). A culture is the combined practices and beliefs of a people. What if God is just the practice of treating the totality of one’s civilization as a single entity, a group noun of sorts?
Morality arises from value-judgments and decision-making processes in human interactions. If all human interactions are considered, in sum, as the actions of a single thing (one’s people, one’s culture, one’s god), then morality IS grounded in God. The Biblical God, then, becomes a metaphor for a culture (just as every other thing that is hard to explain, or explain away, in the Bible, is a metaphor).
I highly recommend reading the Bible with this interpretation in mind. Each time you encounter God replace it with ‘the will of the people’ or ‘my interpretation of the will of the people’, or ‘what I wish the people would will.’*
On this reading, we really are all made in God’s image, by definition.
Of course this also explains why morals are thought of as objective – if you take the human sphere (subjective, but broadly agreed upon) and extrapolate it out to encapsulate the whole universe, then it becomes objective, but only as an argumentum ad absurdum.
*The cases where this doesn’t work are when God is trying to establish the bonafides by which he has the right to be seen as the God of the Israelites, i.e. the early parts of Genesis (which are then referred back to in his dialogue with Job, which is just the people explaining to Job why bad things happen to good people).