To flatten image in Photoshop, do the following steps:
- Select the topmost layer in the Layers panel.
- Hold down the 'Shift' key and select the bottom most layer to highlight all layers.
- Right-click on any highlighted layer and choose 'Merge Layers' or 'Flatten Image.'
- Confirm the action.
- Save your flattened image with a new file name to retain the original layers.