1. Open your image in Photoshop.
2. Go to "Image" > "Canvas Size"
3. Change the units to "Pixels" and adjust the height of your image to make it exactly half of its width. For example: if your image is 4096px wide make it 2048px high.
4. Adjust the anchor according to where you need to add space to your image:
- To add space at the top, place the anchor in the bottom middle position.
- To add space at the top and the bottom, leave the anchor in the center position.
- To add space at the bottom, place the anchor in the upper middle position.
5. Choose an appropriate color for the empty space and use the fill tool to fill the space.