Rasterbator.net (horrible name, although it is memorable) allows you to upload your own photos and turn them into multi-page posters of any size. It splits the image into multiple pages that you then trim and tape together.
Super convenient for anyone traveling to a conference or working in a variety of spaces – just pack the poster in a file folder! Students could print posters from photos they take for a school project. Keep in mind that it will use a lot of ink, depending on your final product size. You can also print in black and white if you don’t have a color printer.