If you think Instagram is just about pretty pictures, you're really missing out on opportunities. Sure, a pretty picture is going to stop your audience in their tracks, but it's not necessarily going to be enough to start a conversation.
Instagram allows you to use up to 2200 characters for your caption. So take advantage of the opportunity to engage your audience through words that complement the photo.
Use your captions to tell people the story behind the photo, invite them to comment or share their thoughts, ask a question or add a call to action. Whatever you do, it will be more effective than letting the picture just stand on its own.