The usual understanding is that we're in the image of God because we are capable of making responsible decisions, can feel love, etc.
In Gen 2, God considers various possible companions for Adam. He rejects the animals, and thus creates Eve. This seems to define humanity in terms of the ability to be companions.
This article on the image of God is interesting
What Does “Image of God” Mean? - Articles. It maintains that in the ancient context of Genesis, being in God's image means being his agent, ruling on his behalf. In terms of Gen 2 that is the responsibility to tend the garden.
None of this applies to a fertilized egg. I'm concerned about implications other than abortion. One is end of life decisions. We are increasingly able to preserve bodies in some kind of life. Should we? Have you seen picture of rooms full of bodies sustained by tubes? Is this the image of God? What happens when we can sustain some kind of life indefinitely?
I understand that we have to be careful about drawing lines. We don't want disabled people treated as valueless, nor do we want to abandon unconscious people. But I think at both the beginning and ending, a functioning brain seems like something we should require in order to treat someone as having full human rights.
Unfortunately this is not a question on which the Bible really speaks. But I don't see any grounds for the assertion that the only possible Biblical position is that any group of cells with human DNA is a human being.