Rhetorical: While verse 19 and a beautiful image (having the image of God engraven upon your countenances) and is a much more pleasant verse to discuss than verse 20, nevertheless the two should be seen together. Alma is creating a contrast, and to separate these verses diminishes the impact that Alma is trying to make.
We must continually remember that Alma is speaking to members of the church here, and is calling them to repentance. Alma must therefore convince those of the congregation who had fomented contention in their own way that they should return to proper actions. He does this by contrasting the two types of believers standing before the judgment bar of God.
There will be those who have had the proper works (symbolized by the pure heart and clean hands) and therefore have become similar to their God (having his image on their countenances). There will also be those who have served a different master. While Alma does not complete the image by suggesting that they will have the image of devil on their countenances, the idea of their close association with the devil is clear, and it is clear that it is based on whether or not their actions followed the principles that they should espouse if they truly kept their covenant with the Christ.
The poetic picture of the image of God engraven on their countenances appears to be unique to Alma. It is possible that Alma is expanding the imagery that Benjamin used, since we know that Alma is referring to the covenant made under Benjamin. In Benjamin's speech it is the heart and not the face that is the focus, but the concepts are parallel:
Mosiah 5:12
12 I say unto you, I would that ye should remember to retain the name written always in your hearts, that ye are not found on the left hand of God, but that ye hear and know the voice by which ye shall be called, and also, the name by which he shall call you.
Of course, it is also very possible that the specifics of the Benjamin text were influence by Joseph's reading of Paul who explicitly uses this imagery:
2 Cor. 3:3 Forasmuch as ye are manifestly declared to be the epistle of Christ ministered by us, written not with ink, but with the Spirit of the living God; not in tables of stone, but in fleshy tables of the heart.