There are to types of posts: Link posts and Image posts.
If you wish to make a link post the first thing you should do isinsert a URL in your post. Then, you can edit the image in the post with the option "Another Photo":
Note: You can only upload images from your computer with this option.
If you wish to make an image post, then you can use our option to turn you link into an image:
Note: This will allow you to delete the URL you have inserted and use only the image in your post.