According to our image policy, images can only be uploaded here when they are necessary to depict something not already represented on the page. A better approach is to see what kinds of pics are needed before uploading them, rather than uploading the ones you have and trying to find a purpose for them.
Where the image comes from doesn't mean it's in line with our policies. There are plenty of images that could come from nick.com or screenshots of the show itself that we imply don't have a need for on here.
It could've been because there was already a picture that showed her as a fully-realized Avatar, or that the images were cluttering the article. We don't need images on every article to depict every individual detail or plot turn.
Korra looks exactly the same after she airbended as she did before. If there is a picture of her once she has achieved fully-realized status, it should be presenting her doing something significant, and not cluttering the article in the process.
Yes, she is fully realized. I never said she wasn't. There can be a picture of her in the Avatar State or energybending only if there is enough room for it in the article. We can't clutter pages with images depicting every event that happened in the show.
I don't know why it was removed. It could've been because the page was cluttered. You are free to discuss the change with whomever removed the picture, and tell them why you think it should be included. But if the picture has been removed, you shouldn't re-add it without discussing the topic.
The code to the right is the code of the picture that I just posted. The code on the bottom is just showing what each part of it is. The "thumb" is always there, but the rest are variables: the name of the image, where it's alligned (left, right, center), the size (ex: 200px, 300px) and the caption if you want one.