The most accurate term is compositing, and the finished result is a composite image. That means combining elements from two or more separate photos into one final picture.

If the blend is made to look continuous or seamless, people may also loosely describe it as image merging. Stitching is sometimes used too, but that term more specifically refers to joining overlapping images—such as for a panorama—rather than combining unrelated scenes from different locations.

So in your case, compositing or composite image is the best term.