1. Open image in Photoshop.
2. Choose Image -> Canvas Size
3. Enter your preferred size under Width and Height. This will vary, depending on the image. Afterwards, click OK.
4. Choose the Eyedropper Tool and click on the orange background. This will enable you to select the background color.
Notice that the foreground color has changed from red to orange.
5. 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.
6. Choose File -> Save.