To add a sticker to a photo on an iPhone, follow these steps:

1. Open the Photos app and select the photo you want to add a sticker to.
2. Tap the Edit button.
3. Tap the Markup tool, represented by a pen icon.
4. In Markup, tap the + (plus) icon and select "Add Sticker" from the menu.
5. Choose a sticker from the default Apple stickers or your custom sticker library.
6. Drag, resize, rotate, and position the sticker as you like on the photo.
7. Tap Done to save your edited photo.

Additionally, you can create custom stickers from your own photos by long- pressing on a photo subject in the Photos app and selecting "Add Sticker" to add it to your sticker library for use in Markup or Messages.