The main post image is very important: it shows on blog post lists. Make your post stand out, but avoid copyrighted images. If not, we may have to delete your post. It is safe to use images from the following websites: unsplash.com, creativecommons.org, pixabay.com, pexels.com, piqsels.com, freeimages.com. Don't necessarily look for something chess related. You can take a poetic, metaphoric approach to your image search. Start from a chess term you're talking about and use it outside of its chess context. You can also use images that you made yourself, pictures you took, screenshots of Lichess... anything that is not copyrighted by someone else.
