PhotoCollage allows you to create photo collage posters. It assembles the
input photographs it is given to generate a big poster. Photos are
automatically arranged to fill the whole poster, then you can change the
final layout, dimensions, border or swap photos in the generated grid.
Eventually the final poster image can be saved in any size.

The algorithm generates random layouts that place photos while taking
advantage of all free space. It tries to fill all space while keeping each
photo as large as possible.

PhotoCollage does more or less the same as many commercial websites do, but
for free and with open-source code.