It’s not difficult to infringe on an Instagram user’s copyright. In fact, sometimes this is down without the person realizing they have infringed on a copyright. The difference between a “public” and “private” label doesn’t grant permission to use or not use an image. It’s just a label that designates who can view the image. It’s not a waiver to use said image. If you don’t have permission, you cannot and should not share other people’s photos. You can only share Instagram photos when it’s allowed by the owner. You own your own images that you create or capture. You don’t have ownership, permission, or license to use other people’s images as you would your own. Be cautious about what you do with photos online.