To add stickers to photos on an iPhone, follow these steps:

1. Open the Photos app and select the photo you want to edit.
2. Tap "Edit" at the top right corner.
3. Tap the Markup button, which looks like a pen inside a circle.
4. Tap the "+" icon, then select "Add Sticker" from the menu.
5. Choose a sticker from the default Apple stickers, your own custom stickers, or app-synced stickers.
6. Drag the sticker to your desired position on the photo. You can resize or rotate it by pinching or rotating with two fingers.
7. Tap "Done" to save your changes.

You can also create custom stickers by long-pressing a subject in a photo (like a person or object) in the Photos app, then tapping "Add Sticker." These custom stickers get added to your sticker library for use on photos or messages. If you want to remove a sticker later, open the photo in Edit mode, go to Markup, and use the options to delete or revert the changes. This feature requires iOS 17 or later, with added enhancements in iOS 18. It works directly within the Photos app without needing extra apps.