Every time I create a TpT preview file, I always have a sketch of it first. I choose appropriate shapes, colors, typeface, font colors, etc.
I do not pick off the wall stuff because I want to pick features that relate to the title (e.g. Natural Disasters) of the clip art set.
I would show some of my new preview files I have done recently:
I am going to create the last new preview file for "Weather Kids" next week. I cannot wait for you all to see it.