Creating a poster is different than making other types of figures, mainly because of the problems posed by the larger dimensions of a poster. A high resolution 36x60" Photoshop file would be too large to manage. The program would probably run very slowly or crash.
What do we suggest? Use a program that links rather than imbeds images within it, such as Illustrator. PowerPoint is NOT the optimal program for making a poster.
See these resources for information on creating and printing posters:
more information on using Illustrator (placing images, adding text),
see the Illustrator page.