I think you want to go one of two directions one this:
Pick an image that only needs one major adjustment, which is the one you're showing. If you're demonstrating cropping, pick an image that's good in all respects except for cropping, so that your example focuses only on the crop. I'd go in this direction if you're demonstrating only one or two techniques.
If you're doing more than one example, I think there's value in re-using images so that you can see the effect of a variety of edits on the same image. This makes the most sense when you're comparing techniques that are largely alternatives. In the cropping example, you could show the same image as a landscape crop vs. a portrait crop, for instance, or use it to illustrate the law of thirds vs. the golden spiral, and so on.
[edit per Q in comment]
I'd expect something on the order of one image per class of edit, so one with clear, vivid colors could be used to show hue, saturation, B&W processing, etc. If you find an image that can also be used to show composition edits, or whatever, there's certainly no harm in reusing the same image for that, too.
In the example in your original question, one of your edits is cropping. While any image will show how cropping works, I think you're ideally looking for one that shows why you'd want to crop one way vs. another, so it would be great to have an image that (1) benefits from cropping, and (2) takes on a different character or emphasis as a result of cropping -- after all, that's why you're cropping, right? Maybe you don't have a single image that works for both portrait and landscape crops, and if that's the case, you could show how one image is better-suited to a landscape crop, and another is better-suited to a portrait crop, and so on.
In the case of WB adjustment, you could show WB adjustment as a fix for a poor "as shot" WB (in which case you'd want an original that shows an incorrect case), or you might also illustrate WB adjustment as an artistic edit (ex: "see how the picture takes on a different look when the WB is warmed slightly").
For each of these cases, you want the example to be an "aha" moment for the reader where you're leading them by example. Point #2 above, though, suggests that if you're illustrating "n" examples, you might not need "n" original images. In fact, most books and tutorials I've seen come with a small assortment of example images that are used throughout the documentation, and in fact, are made available to the user so he can follow along. In some cases, I've also seen the "before" and "after" files made available to users so they see the results of the edits on their own screens.