Many artists don't have all of the elements showing on the first listing image, so you can't see at a glance whether your desired object is included. It may look prettier to have a simple shell with "Let's go to the beach" written artfully in script on the first listing thumbnail, but it's also confusing. I might skip it thinking that's the only art included. Your thumbnail can act as visual "tags" for someone scouring search results.
Here's one that's done right, in my opinion:
There's a title for reference (and to help differentiate it from other sets in her shop), the pictures are artfully arranged, and you get a good sense for what kinds of unique elements are included. I love that the flower shop and flower truck are front and center--those aren't as commonly found in watercolor clipart sets. Flowers, on the other hand, are super common and don't need to be featured as prominently.
Here's a mockup I created of one that's not very helpful:
This might be appealing and give a certain vibe, but it doesn't really tell me much. At least it indicates there's a set of clipart elements (the word "collection"), but is it purely a floral set? Is it all about gardenias, as the one pictured, or are there a variety of flowers? Does it have beachy elements? Tropical? People? Clothing? Food? Even if only some of the included images can fit, at least cover some of the variety of topics so shoppers get a more complete picture at a glance and be drawn in.
Here's a quick example of how I might add more so it tells a fuller story. It's not as aesthetically pleasing (to me, at least), but sometimes you have to think practically. (Remember that this is a fake mockup, so I'm not saying those pics really go together for a great set. 😆)
If you sell clipart, be sure to check out my other related posts and also introduce yourself! I'm always interested in new clipart resources. :)