1. Create a new document 850 X 625 pixels
2. Take Flood Fill Tool... fill the image with black color
3.Shapes Rectangel ... make a rectangel about your image
4. Go to Styles ... Category Blend ... Styles: Background
5. In the Layer panel ... Right-click... Rasterise
6. Open the file: Blanding ... right-click the picture... Copy
7. Go back on your work picture... right-click it ... Paste
8. In the Layer panel ... Right-click... Rasterise to mask
9. In the Layer panel ... Right-click... Merge down ... now it looks like this: