Here's the commands needed to convert a picture called "mypicture.jpg" to a big poster in a Postscript file called myposter.ps:
convert mypicture.jpg mypicture.ps
poster -mA4 -p4x4A4 mypicture.ps >myposter.ps
The parameter -mA4 selects the paper size you want to print the poster with. The parameter -p4x4A4 sets the poster size as being 16 A4 pages.
You can print using whatever printing application you like, be it lpr on the commandline or a graphical application such as Evince on GNOME systems (such as the Ubuntu Breezy system I am using).