for occasional printing of a poster using multiple images on a standard printer.

Set up grid or guides at appropriate size (example is 8 inch x 10 inch)
Set the marquee selection tool to a specific size, with a little added for overlap (8.25in x 10.25in)
These sizes must fall withing the printable area of your printer
Set Snap to grid/guides
Select each grid box and copy contents of the original image to a new layer.
Print each layer on a separate sheet of paper.
Trim each picture. The right and lower edges will allow a little duplicate overlap for assembling the images.