In Photoshop, select 'Image' (top bar), within that menu you will see 'canvas size'...... enter your desired width that you want to increase it by.
This will leave you with two white blocks on either side of your image.
Then select the 'Colour Picker' (left side the small dropper symbol), click on the area behind the girls.
Select the 'Paint Bucket' tool.....and just click on the white areas and it will fill them in.
Neil.