Printing a large image on multiple sheets
The free app PosteRazor is easy to use and works with any .jpeg image. However, in my hands the printed image was not the actual size, it's about 5% too small. This would not likely be a problem for a poster, but could be very bad for a pattern or engineering print.
Adobe Illustrator (at least version CS4) can do this, and in my hands the print image is exact. In the "Print" dialog box select the "tile" button. The large image and the pages that make up the tiles will appear in the window. You need to select an "overlap" value so the images can be joined after printing. I have found a value of 24 mm more than sufficient, but you need to experiment with your printer/driver.
Adobe Illustrator can open (or import) .jpeg or .ps files. I don't think Photoshop can tile large images (it should IMHO).