Confused with injection and surjection on ring homomorphisms

If $\phi: R \rightarrow S$ is a ring homomorphism, then $\phi':R/ker(\phi) \rightarrow S$, defined as $\phi'(r+ker(\phi)) = \phi(r)$ $\forall r \in R$ is an injective homomorphism.

This is true because $r_1+ker(\phi) = r_2+ker(\phi) \iff r_1-r_2\in ker(\phi) \iff \phi(r_1-r_2)=0 \iff \phi(r_1)=\phi(r_2)\iff \phi'(r_1+ker(\phi))=\phi'(r_2+ker(\phi))$, and so $\phi'$ is well-defined and is injective. (It is also a homomorphism - not relevant here). Also, $Im(\phi) = Im(\phi')$ - I can't seem to prove this.

Now, being $\phi$ an homomorphism, $ker(\phi)={0} \iff \phi$ is injective.

The question is: if I have an homomorphism from the quotient ring $R[X]/⟨f(X)⟩$ to a certain $S$ ring, where $degree(f(X)) = n$ is an irreducible polynomial in $R[X]$, what can I conclude about those two being isomorphic or not?

P.S.: I had this question whilst reading Is this quotient Ring Isomorphic to the Complex Numbers and the answer by Bill Dubuque (how can we know that the $f$ mapping is onto?)

• Note that the result you are referring to is not about $S$ but about $\mathbb{R}[X]/I$ where $I$ is generated by an irreducible polynomial – JJR May 25 '17 at 9:49

To see that the image of $\phi$ is the same as the image of $\phi'$ more formally, pick an element $s\in S$ in the image of $\phi$. Then there is some $r\in R$ such that $\phi(r)=s$.
Okay great, so what. Now what do you think in $R/\text{ker}(\phi)$ will map to this $s$? The most obvious thing: just the coset of $r$ above. More concretely, we compute $$\phi'(r+\text{ker}(\phi))=\phi(r)=s$$ so that the $\text{im}(\phi')$ is atleast as big as that of $\phi$
And the image of $\phi'$ can't be bigger because any $s\in S$ hit by some $r+\text{ker}(\phi)$ under $\phi'$ is actually hit by a representing $r$ under $\phi$!