You have a good start with love being constant and consistently sublime. That is indeed what Sonnet 116 is all about. Check some of the Enotes links below; they should help.

You can expand on the concept of love if you want and in this sonnet, it is particularly romantic love. But in terms of the imagery of this poem, love is compared to some thing, abstract or physical, which can never be tarnished, diminished, or altered. This “true” love, since it is so constant and so perfectly preserved has a unique quality in that it is invincible but it is not described as powerful in the sense of being destructive. Being an abstract quality, love is also manifested physically (making love and in other gestures and words between lovers); thus, the speaker says that it “looks on tempests and is never shaken.” Not even a hurricane can move it. Speaking of love as something that is also like a guiding light, the speaker says,

It is the star to every wandering bark,
Whose worth's unknown, although his height be taken.

Here, Shakespeare uses a star which is a physical object but one which is in the heavens. This gives it a quality of being both physical and spiritual. It also gives love a sense of being seen but not touchable; visible, but light-years away. We can see it and feel it but we can't touch or harm it. 

Whenever you're dealing with imagery in poetry, don't be afraid to go outside the poem with additional analogies and metaphors. Expand on the imagery. Use physical and abstract things if you can. What else is like the love that is described here? What else is visible but untouchable? What can't be moved or destroyed? What is constant, or more accurately, what is constantly good? What images transcend space and time? Well, this last question is a bit tough. So, start with images that share at least some of these qualities. Then, you have to answer the “so what” question. Love transcends space and time. So what? Why is that good? It seems like a stupid question to ask, but it needs to be answered.