MS Publisher has a poster function. I have use it before to create tiled images. Most of the time Publisher comes bundled with MS Office so you should be able to find it.

To use it, I opened Publisher to a blank poster. Resized the page to the size I want it to be, then added an Image to it (I have used large 5-8 meg sized jpeg and its worked just fine). The tiles will automatically have some overlap, which you can adjust if you want.