i have a image but it is not mine, should I post it on here? I am not sure about who have the copyright of that pic.

It depends on what it is.

If its just a random sprite some random dude made, dont post it, its like theft, even if you do say you didnt make it.

If its like a new image from the Pearl/Diamond range, post it in that board.

I dont think you shoudl post something that isnt yours though unless its really cool/funny and you think everybody should see.

*nods* If you didn't make it and its a sprite or a drawing, don't post it because its technically theft.
You could get permission from the artist to post it, but artists tend to not let people who aren't themselves distribute their work.

I thought the fanart section was supposed to be for the work you made yourself so you could show it off, personally..