You can add a border to an image in Photoshop by following these steps:

1. Open the image you want to add a border to in Photoshop.

2. Select the layer containing the image.

3. Click on the Layer Style button at the bottom of the Layers panel, or go to Layer > Layer Style > Stroke.

4. In the Layer Style dialog box, set the stroke width, color, and position as desired.

5. Click OK to apply the stroke to the image.

You can also adjust the stroke options by double-clicking on the Stroke layer style in the Layers panel, which will open the Layer Style dialog box again. From there, you can make changes to the stroke width, color, and position, as well as other options like opacity and blend mode.