Hi @NotBob
The sketchbook category is meant to hold a series of artwork, not one-by-one. Think of it just like a physical sketchbook. You might have one on your shelf that is dedicated to a particular year, or maybe another dedicated to hand studies or portraits, but usually we don’t have a sketchbook with only one image in it.
You can change the thumbnail anytime you like and as many times as you like. Or you can leave it the same, it’s up to you. Each time you add a new image to your sketchbook by clicking the reply button and each time someone comments by clicking the reply button, your sketchbook will be highlighted to the membership.
Nohara’s sketchbook is a good example. Each time they post a new image I see it pop up in the new topics feed.
Did that help at all?