Ah! I get the auditing, it's just good to know who is or isn't interested vocally!
I do base things out of drawing pretty clearly, but it's not a requirement really! It's about building a way to plan images of all mediums, which can lean as heavily on reference as you need it to be. It's easier if you are sorta comfortable at drawing, but it doesn't have to be like that!
Thumbnails pretty much only have to be at the level below, and from comps onwards if you wanted to do more graphic design/signature based stuff
(we'll call them digital collage in illustration terms) you could begin doing drafts with the actual images you are thinking about using. And even if you did want to do it on drawing art or pixel art, the important part is the conceptualizing and composition, that's why I don't require a final image.