Sadly, users cannot add an image to an already published Instagram post. If another image is to be added, the original has to be deleted and a new upload must occur instead.
To add an image to the post:
1. Copy your caption: Always copy your caption so that it is not deleted in case you forget.
2. Delete the post: Tap the three dots in the upper right corner of the post and hit delete.
3. Re-upload: Make a new post with your caption and attach all the images you want.
I know it can be kind of annoying, but in the end, that post will be just how you envisioned it.
