Besides Photoshop, what are the best software/online services available to take any number of images, and assemble that collection into a photo collage that contains the photos therein, but also each photo can be resized as needed/desired, and also the user can define the layout of the template, what the dimensions of that collage will be, along with the ability to customize backgrounds, spacings between images, etc.
I would give this a try and see how it works (never used it BTW): http://www.fotor.com/features/collage.html
Here another: http://www.picmonkey.com/collage
Have used this program on my mac and it is pretty good though basic: http://www.collageitfree.com/
Hope this helps!