What kind of presentations? Aperture can make slideshows and smart webpages with image galleries, but for presentations involving text and for posters you would better use word processing software, like Keynote for presentations and Pages for posters (or Powerpoint and Word, Open Office to name other commonly used programs).
For posters: Posterino is a good choice, imo.
How often do you need this facility?
Posterino is better than some of the ones named above, more feature-rich & certainly better than WP apps to accomplish this! There are other competing apps, as well. At least until such feature finds its way into Aperture.
Actually, you can do quite a bit of collage style layout right in the Book feature ... just create a new book with a custom page size for the finished presentation product you desire ... then print the page directly or export the page as a jpeg or PDF ... the exported files could be used in any other app ... Slideshows, Keynote, etc. ...