I don't have images to share, as they are often specific to the customer/company. But I have some tips that might be useful:
Always follow the visual identity of the company you are working for. This helps users understand where they are and makes the experience more intuitive.
Choose simple images, without many details, as they will be displayed in a very small size and will lose visibility.
Qlik app and sheet thumbnails must be 8:5 images, otherwise they will be distorted.
For app thumbnails, you can think of colors by streams.
For sheet thumbnails, a printscreen of the sheet itself works very well, as the user naturally tends to store a "photo" of the screen in their head, so when viewing the thumbnails they will navigate in a very intuitive way.
Furthermore, in this link you will find a tool for creating thumbnails using the icon + text pattern. Although extremely useful, for future maintenance (creating new tabs, or changing names), I suggest you use some simpler software. As incredible as it may seem, Power Point ends up serving me very well. There I can modify the slide size to meet the 8:5 ratio, I can easily add and position texts and icons, and I can even export all slides as images (png). Furthermore, pptx serves as a repository for thumbs, making it easier to maintain them in the future (I've included an example in the attachment).
