Photo Collage

There is no add-on for iPhoto to create collages. However, there are numerous apps in the App Store for creating collages. Do a search for "collage". Many are free.

Or you can use Pages to create a collage. First create a custom page size in the print window to match what size you want. Then open Pages and create a new document with that custom size. Then use the media browser to locate and add the desired Photos to your page. Once on the page the images can be resized, rotated and placed where desired, many overlapping. Export the finished page as a PDF file. Open it with Preview and convert to a jpeg file. Then you can do what you want with the jpeg file.

Once can create large collages with Pages like this 20" x 30" poster collage: