Open the photos you will use for the superimposition in your image-editing program. Popular programs used for this process include Adobe Photoshop, Corel Paint and PhotoStudio.
Make a selection of the person's photo so you can copy it from the original photo to your destination photo. This works similar to physically cutting out the actual image of the person in a photo, then pasting it to an entirely different photo where you want it to be superimposed. Although image-editing programs vary in the actual button or feature they use for this process, they essentially have the same function of selecting the contour of the element on an image so you can transfer or copy it to a separate photo. If you have more than one person to add for the superimposition, select each one individually for better control and accuracy of the selection.
Copy-and-paste the person's photo onto the other photo where you want it to appear. This will create a new layer on the destination photo, which is right on top of the original image.
Adjust the size of the person's photo to make it believably a part of the other photo's background. Use the "Scale" or "Resize" function of your image-editing program for this type of adjustment. The actual button name and location of this function depends on the program used.
Adjust the lighting attributes of your superimposed image so that it better blends on its background, based on how the main light source such as a lampshade, streetlight or the sun falls on the background photo. Use any popular tool from your image-editing program such as "Highlights" or "Dodge" for this process. This allows you to lighten up certain parts of the person's photo so that it looks like the light falling onto the face or body comes from the same light source as its new background.
Adjust other image attributes including the "Opacity," "Contrast," "Brightness" and "Saturation" so that the person's photo looks as if it's really part of the background photo. You can also use any of the available filters under the program's "Filters" menu to further enhance the look of the superimposed image. For instance, you can add a warm filter so that a person originally shot in a cooler background can look warmer when superimposed in a landscape that features the sunset.
Save your new photo of the background with the person's shot superimposed on it.