In art in general, a ‘montage’ is the creation of a composite of multiple images or pictures which are mounted together so they blend together in an artful way. The result is a new synthesis which may create a picture in its own right, create a story, or convey artistic meaning in some other way.
A photomontage is a particular form of the art of montage. To be properly called a photomontage the creation of the final composite image should be through the use of ‘photographic processing’ – originally a chemical process. Considering photography in its more recent form, digital processing would probably qualify too.
Since photographs tend to have a regular, orderly shape, photomontages tend to be organized. However, the use of a photomontage as an art form is frequent. So the arrangement of images is a matter of art. A photomontage may be logical in order, as in a story. They may be subject groupings. They may be unrelated or have a grouping apparent from the subjects of the individual images.
A photomontage will be a composite of a multiple images that in some way satisfy the photographers need to express themselves using more than two images. Other terms could also be used for a few pictures combined as two or more on one image (see related links below).
The use of the term ‘collage’ is often used in common speech to be interchangeable with photomontage. However, the history of the word collage suggests that there is a distinct difference. A collage is created using traditional ‘scissors and glue’ methods. A photomontage is created as a result of a photographic process.