1. Open image in Photoshop.
2. Select the Crop Tool and remove the white border.
Note the difference.
3. Go to Image -> Canvas Size.
4. Enter your preferred size under Width and Height. This will vary, depending on the image. Afterwards, click OK.
5. Use the Rectangular Marquee Tool to partially select the red section at the bottom. Copy and paste.
6. A new layer should have appeared. Select the Move Tool.
7. Extend the red section by moving the layer to the right.
8. Click Ctrl + T to transform the layer. Further extend it by dragging its right side. Afterwards. Press Enter.
9. Do the same with left side.
10. Create a new layer and place it under the other layers.
11. Select the Gradient Tool. Click the gradient to edit the colors.
12. Double-click the 2 color stops to edit. Make sure the 1st color stop is the same color as the bottommost part of the image. The 2nd color stop can be a bit darker than the first. Click OK.
13. Use the Rectangular Marquee Tool to create a rectangular selection at the bottom of the image.
14. Go back to the Gradient Tool. Drag to fill the selection. Click Ctrl+D to deselect.
15. Create a new layer and drag it under the other layers.
16. Choose the Eyedropper Tool and click on the black background. This will enable you to select the background color.
17. Select the Paint Bucket Tool and click on the transparent section of the background.
This should fill up the rest of the background with the same color as of the original image.
18. Choose File -> Save.