Trim pictures so that they are different sizes and shapes. Some photographs can be the traditional square or rectangle, but you can also make some photographs into a circular or diamond shapes. Cut some pictures smaller than others, have a medium size, and include a few large ones to provide diversity.
Frame the pictures in assorted frames. A wall collage can be created using all the same frames, but then you do not end up with a contrast. Assorted frame ideas include using some frames that are metal, some that are wood, some that are made out of twigs or covered with fabric, and some that are brass.
Lay the framed pictures on the ground or working surface. Mix them around to get a feel for where the pictures and frames should go when hung on the wall to make the collage. Avoid keeping the pictures too organized. For example, don't put all the large pictures on one side of the collage and the smaller ones on the opposite end. Instead, place one large picture frame next to a small one, with a medium one spaced someplace else and another large one. This will give it the appearance that the framed pictures were randomly selected.
Decide how you want the collage to flow. For instance, some wall collages with different size pictures are more horizontal than they are vertical, and vice versa. Create a cluster effect with the framed pictures for a collage with various sizes so that the collage is neither too horizontal nor too vertical.