When you are on the go and don't have time to make a complete post, go the "Quick Post".
You will be asked to select Camera or Gallery.
It is your choice, but usually you will choose Camera. Take the quick snap.
Now you have to choose either "Retake" or "Use Photo".
You will receive a pop-up which says, "Later" or "Now".
If you choose later, the screen will close.
The photo is stored until you have time to populate the post.
When you have time to populate the post, go to your settings (i.e. hamburger, top right corner of you phone)
Click on "Quick Post" and there is your photo that you took earlier.
Click on the three dots (top right) to start populate your photo.