"Na'aseh adam b'tzalmuseinu k'dmuseinu" (Bereishis 1, 26). Rav Wolbe (Shiurei Chumash) quotes the Nefesh HaChaim who explains the concept of "tzelem Elokim." Firstly, he points out that in this context, the word "d'mus" cannot be translated as "image," because Hashem has no image at all. Rather, "d'mus" is a derivation of the word "domeh" which means comparable, for in some way man can be compared to his Creator. So too, "tzelem" which is a synonym for the word "d'mus," has a similar connotation.
Secondly, he notes the Torah's choice of "tzelem Elokim" and not "tzelem Hashem." Elokim is the description of Hashem as "Master of all strengths Who is able to do anything." In other words, Hashem created everything ex nihilo. He gives the strength to everything to allow it to exist. Should He decide, for but a moment not to give any item its "strength," it would instantly cease to exist. It is in this respect that man is somewhat comparable to his Creator. Hashem ordained that with every action, every thought, and every word, man affects one or many of the myriads of the higher worlds. A positive action gives strength to the existence of these worlds, while a negative action in effect destroys them. As we say after Ein K'Elokeinu with reference to talmidei chachomim, "Do not read ba'nayich (your sons) rather bo'nayich (your builders)," because they are the builders of the spiritual worlds. In light of this concept we can explain the Mishna in Pirkei Avos (2, 1) "Know what is above 'from' you. . ." One must know that everything in the spiritual worlds above him is "from him" - i.e. is directly affected by his actions, thoughts and speech. Therefore, a person should not think, "What could little I do already in this world?" Even though it might be indiscernible to him, everything he does has the ability to create or maintain the higher worlds.
Rav Wolbe comments that most of us live our lives in consonance with what the Nefesh Ha'Chaim writes is the incorrect perception. We have no idea of the awesome potential of man and hence, we conduct our lives with a constrained mindset. At the end of the parsha (ibid. 5, 1) the Torah is referred to as "the book of mankind" (see Ramban), since one who lives his life in accordance with everything written in the Torah, has revealed man in his truest form. In the eyes of the Torah, greatness is not measured by how many grandiose buildings one has built or lawsuits he has won. It is measured by how many times a person overcame the countless temptations that present themselves throughout a person's life.
The Alter of Slabodka spent his entire life preaching about the greatness of man in general and Adam HaRishone in particular. His discourses had a major effect, and numerous great leaders emerged from his yeshiva (Rav Aharon Kotler, Rav Avrohom Grodzenski, Rav Yitzchok Hutner, Rav Eizak Sher, Rav Meir Chadash to name a few). If we want to become big, we must begin by thinking big, and the first step is recognizing the awesomeness of being a tzelem Elokim.