Two of my ideas for why B'tzalmenu cid'mutenu is written in the plural:
1. Hashem is talking to us, even before our creation. We are created in our own image, in the image G-d imagined for us. We are everything Hashem imagined for us, and it is important we recognize this. That our creation is loved by Hashem. And that we are partners in creation with Hashem, both by how we contribute to the world in general, and also if we have children, by the fact that we are creating new people that will take on what we show them and be incredibly influenced by how we raise them.
2. Hashem is talking to the animals. We are made both in the image of Hashem and the image of the animals. We both have the capacity for great spirituality and great bittul of spirituality, through prioritizing physicality. When we conquer this part of ourselves that is more like animals, and keep it in check, we are moving closer to our purpose on this Earth. Unlike animals, we can elevate the physical to something greater, for example by saying a bracha and feeling grateful to Hashem for the food we eat. In this way, we have dominion over the animals.
(27) And God created humankind in His image,
in the image of G-d He created them, male and female he created them.
b'tzalmo here, going along with explanation one, could mean in G-d's image, but then that brings up the question of why it is repeated. Another explanation is that this is emphasizing that as humans we are also created in our own image.