You do (at least as between you and Instagram—your employer may have ownership rights in certain situations)! Instagram does not claim ownership of any content that you post.
You do grant Instagram very broad license rights: a non-exclusive, fully-paid and royalty-free, transferable, sub-licensable, worldwide license to use content that you post. This license grant means that you have given Instagram the right to use any of your photos for free, for any reason, anywhere in the world. Instagram can also give those rights to a third party.