The collage? You take the photo that the contest owner provides and add whatever elements you think will produce an outstanding compilation.
Some contestants just add other images to the original. It's great to have a transparent image.
However, you need a photo editor. There are numerous programs that help you edit photos, such as photoshop.
I usually use Canva and upload the photo and images. I explain my procedure in the post. Sometimes, I'll only use part of the original contest photo. I find Canva to be the easiest for me.
There are some contests who are expert at gif making and combining the elements. They produce beautiful collage images.
In the LMAC Community, there are tutorials from a member that explains how to accomplish it.