1- Open Affinity Designer.
2- Open the first picture you would like to change the background of.
3- Go the layers palette, copy and paste your image.
4- Delete the original image.
5- Using the pan tool draw around the part of the image you would like to keep.
6- In the layers panel drag the background image into the curves layer.
7- Change the stroke to no colour.
8- Next go to File > open.
9- Select the file you would like for your background. Right click on it and copy.
10- Right click on the image and paste the background image.
11- Go to the layers palette and drag the background image you added below the curves, so it is behind you original image.
12- Select both the curves and background image and select the Fx option. A new box will appear.
13- Finally, you can adjust the blur and other effects to create different looks. I used a glow effect to place a haze around my image.