Olga's Gallery. For a full colour version of William Turner's, "Rain, Steam and Speed - The Great Western Railway", see http://www.abcgallery.com/T/turner/turner26.html.) However, a thumbnail sketch is not merely a reminder of the painting, it also helps you to analyse it and understand its composition. For it to be successful, you have to think about what you are doing, about what it is really important to show, yet it is also fast to do!
In order to improve your understanding of a piece of art work, it can also be useful to make a large, detailed study of it (as in my study of the horse cave painting). But sometimes this is unnecessary, even a waste of time, when (in some cases) a much simpler thumbnail sketch would give you all the insight you need and would get across whatever point that you wish to make!