"So, what is a digital collage?"
Here is the definition of collage from Wikipedia:
A collage (From the French: à coller, to glue, French pronunciation: [kɔ.laːʒ]) is a work of formal art, primarily in the visual arts,
made from an assemblage of different forms, thus creating a new whole.
A collage may sometimes include newspaper clippings, ribbons, bits of colored or hand-made papers, portions of other artwork or texts, photographs and other found objects, glued to a piece of paper or canvas.
The origins of collage can be traced back hundreds of years, but this technique made a dramatic reappearance in the early 20th century as an art form of novelty.The term collage derives from the French "coller" meaning "glue".[1] This term was coined by both Georges Braque and Pablo Picasso in the beginning of the 20th century when collage became a distinctive part of modern art.[2]
So, think Picasso, or to simplify, just follow the rule that I use when I make a paper or digital collage:
Everything must touch.
Unlike Scrapping, or even my beloved Art Journaling, every item on your page needs to touch another- this is what builds cohesiveness and connects the items together. Of course, following a theme you have chosen helps.

The cards in the ad above are examples of paper and digital collages I have made. You can't see it, but the heart and disk are touching the rest via a string.
OH! and the prize??
There is a surprise kit for everyone who posts a page!! The kit will be given out on Monday.

And a winner gets the GC!