Open (click "File" then "Open") both digital images in Adobe Photoshop CS. Save copies (click "File" then "Save As") of both. Adjust the Levels (click "Image" then "Adjustments" then "Levels") of both.

Resize both images so they are the same resolution (click "Image" then "Image Size" and type 300 in the Resolution box). If you intend to print the final image, the Resolution should be at least 300 dots per inch, or 72 dpi for Web use.

Select the part of the image you want to keep with the Magnetic Lasso (click and hold the "Lasso" tool in the side Toolbar to get the "Magnetic Lasso"). You can take a person out of a crowded city street and put her in a park, for example. You will need to be careful to get the whole subject. Feather it (click the "Feather" box in the top Toolbar and type 5 px) to at least 5 pixels to give it a softer edge.

Copy (Command-C [Mac]/Control-C [Windows]) the selected portion of the image. Then go to the second image and Paste (Command-V [Mac]/Control-V [Windows]) it. You can move it around with the Move Tool (click the Pointer Arrow at the top right of the side Toolbar).

Adjust the size and aspect of the pasted image with Transform (click "Edit" then "Transform" then "Scale" or "Skew"). When you get it to the correct size and shape, double-click to Commit (or click the check mark in the top Toolbar). When you have the image where you want it and are ready to print, Flatten (click "Layer" then "Flatten") and save.

Add other images to the same background the same way. You can take an open field and fill it with people, if you like, or take a skyscraper and put it in the middle of a rock quarry. Be careful of shadow angles. If you need to add a bit of a Drop Shadow use Layers (click "Layer" then "Layer Styles" then "Drop Shadow" and use the angle wheel to make the shadow the same angle as the background or lengthen or shorten it with "Distance").