If you need to print images that are as large as your wall, you will obviously not be able to do so your printer can only handled A4 size pages. However, if you can "Tile" sections of your image across several A4 pages, you can place them together and stack them across as large a wall as you need. Check out the website Blockposters.
This website lets you upload an image and select how many pages wide you want your image to be. It then slices the image and creates a series of PDF files that you can print out to generate the various sections of your image. Finally, all you need to do is paste them in the correct order on your wall and there you have a massive poster.
In Blockposters the maximum file size is 1 megabyte, so you might want to split a large photo in parts while create the final large poster.