1) To begin with, open both images in Paint Shop Pro. You might need to make some slight adjustments to these, for example, I flipped the background photo horizontally using Image>Mirror (Ctrl+M) so the window detail appears uncovered.
2) Copy the original picture Edit>Copy (Ctrl+C) and paste it over the new background using Edit>Paste as a new layer (Ctrl+L). The image will now appear over the top of the background.You can now close the original (don't save) as you won't be needing this anymore.
3) Open the Layers Palette using View>Palettes>Layers (F8) and select New Mask Layer>Show All from the drop-down menu in the top left hand corner of the Layers Palette. The new mask will allow you to hide parts of the picture you don't want to be visible (in this case, the background of the top layer), without making any permanent changes to the original.
4) Now you can paint out the original background. Select the Paint Brush tool and open the colour palette by selecting View>Palettes>Materials (F6). Choose black by dragging the selector into the top left hand corner of the Colour Palette.
5) Select an appropriate brush size and reduce the hardness of the brush to give it a softer edge. Now select the Layer Mask on the Layer Palette, and paint over the picture. You will see the image underneath start to appear through.
6) As you get to the smaller details around the edge, select a smaller brush and zoom into the picture to make it easier. If you make any mistakes select the white and paint over any mistakes.
7) Now you can apply effects in order to make the image appear more natural. For instance, you may want to blur the background slightly to make it look less in focus. For this, select the background layer from the Layer Palette and use Adjust>Blur>Gaussian Blur and select the appropriate radius. I found 5.0 was suitable in this case but that depends on the picture. You can also change the colour balance by going to Adjust>Colour>Red/Green/Blue, and adjusting the sliders. This will balance the colour tone of the background and foreground in order to achieve a more natural looking result.
8) Once you are satisfied with the picture, flatten the layers by selecting Layers>Merge>Merge All (Flatten). The two layers will now be merged into one file.You may also want to save a version with layers so you can change the background of the image to a different one in the future.