The rules of the English language. They need to be applied when reading the Bible. The Bible means what it says and says what it means. My first Bible teacher said (quoting Popeye, one of the worlds great philosophers by the way), Just read it the way it was wrote. It helps to have a dictionary. The meanings of words are important. A dictionary is not enough though. English has rules and the rules give meaning. English uses punctuation which has rules and gives meaning as well.
This is important when trying to feather out the concept of the image of God when Genesis 1:26-27 talks about Adam being created in our [God's] image. Pronouns are important as well. Our and us are significant in the context of that passage.
The punctuation of importance in these two passages is the colon and the semi colon. A colon separates two thoughts in a sentence when the first thought is complete and the second thought illustrates the first. A semi colon separates two thoughts in a sentence when the first thought is complete and is not dependent on the second thought but the second thought is dependent on the first and adds to the first thought. This is of vital importance in understanding the meaning in the passages I will use.
Genesis 1:26-27 states:
[26] And God said, Let us make man in our image, after our likeness: and let them have dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth.
[27] So God created man in his own image, in the image of God created he him; male and female created he them.
Go to the colon in verse 26 and look at the thought before it. There are two important points. The first is that God uses the plural us and our to describe Himself. This establishes God as a plurality: a single entity with multiple identities; a trinity perhaps. The second point is that God made man in His image.
Now, look at the thought after the colon. The idea presented, by use of the colon, futher illustrates the thought before it, In our image, after our own likeness." What is illustrated is giving man dominion. Dominion, therefore, is the aspect of being in the image of God that is offered by this verse.
The next aspect of the image of God is illustrated by verse 27. Find the first semi colon and look at the thought presented before it, the creation of man in the image of God. What comes after, by the rules of the English language, further adds to that point. The phrase following the semi colon establishes that man was created as a plurality, male and female, thus mirroring God as a plurality, which was established in verse 26.
My conclusion, by looking at the text without any inference but using rules of grammar, is that being created in the image of God means that man mirrors God in having dominion and being a plurality.