this is an amazing.
that's an amazing .
it's an amazing.
whats the difference ?

Let’s make them into full sentences:

This is an amazing picture.
That is an amazing picture.

Now, in Grammar Monster, there is an introduction to 'this/that'. Read the information there, and let me know.

www.grammar-monster.com/glossary/demonstrative_pronoun.htm

got it .. thanks ..
can we say "its an amazing picture" ?

When we say, "its an amazing picture", we have used a contraction of 'it is',

We show that by writing:
it's an amazing picture

Notice that " it's " has an apostrophe.

" its " is what we use - just like, 'his, her" - to show possession:
The dog was wagging its tail.

There is more about this on Grammar Monster at :
apostrophe