Question

1. Sure. Here, take a look.
2. It's a picture of rocks, right?
3. Not only rocks, Tom! Look closer. There are eggs.
4. Wow, I can't believe it.
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Q1: What is the full form of "Not only rocks'? a or b?
a. They are not only rocks.
b. There are not only rocks.

Q2: Does 'rocks' mean big stones or small stones?

Answer 1

Q1 - "Sure" is certainly not a sentence. In informal, vernacular speech, we use it all the time, but it's a grammar shortcut. Otherwise, your sentences are grammatical.

The definition of a rock is usually a large mass of stone making up a cliff or an outcropping, etc. We tend to be imprecise and apply the word to anything larger than a pebble, though. It would be better usage to say "stones" if they're no bigger than eggs.

Either "they" or "there" is acceptable. I prefer "there" since we're referring to a picture, which is in a specific place, as in "over there". But, referring to a group of objects, "they" is used.