1. Open one of the image sections with MS Paint. It's most convenient to open the top left-hand image first.
2. Increase the canvas area to a size big enough to hold the whole image after they are joined. You can do this either by pulling at the corner of the canvas area or by entering the desired size from Image-->Attributes
3. Go to Edit-->Paste From...
4. Select one of the other image sections by double-clicking or click and open. The second image will be pasted on the first image.
5. Before de-selecting the pasted image, click inside the selected area and keep pressing the mouse button. Now, without letting go of the mouse button, move the second image where you want it to join the first image.
6. Repeat the process for the other sections.
There may be utilities to do the job automatically.