This is good. Upload a photo and make it huge, then download the pdf files to print and stick together. The only drawback is you are limited to 1 megabyte for your original photo. Here is a poster of my cat which I experimented with. You can make it as many A4 sheets as you like. The generator gives you a rough guide to how big the finished poster will be.
Click here for the poster maker
If you don't have software that will resize your original image to below 1 megabyte then search online for a free image resizer, use websites like http://www.photobucket.com/ or use 'Paint'. Here are some instructions for resizing using paint.