you need to bear in mind the order you place your pieces and the bias binding - I usually lay mine out and then number them especially if it is a busy design and I would have thought if the whole backing fabric is going to be covered then it wouldn't matter what you use but if you want a border round it then use backing fabric that would be suitable for that as you don't want to put on a separate border as this could cause problems with the final picture - I tend to use a lightweight fusible web for the picture pieces and then machine or hand stitch down the bias which will also catch the picture pieces in place