When posting on desktop, click the "Flair" option, then select your residential area. If adding a flair after you've already posted your images, click on the three dots at the bottom of your post, and select "Edit Flair".

When posting on mobile, click the "Add Flair" button, right beneath the title. If adding a flair after you've already posted your images, go to your post and click on the three dots on the top right, then select "Add Post Flair".