I found a site with images that are public domain but they charge a few hundred dollars for subscribing every month.
I think a bit of searching on the internet might come up with free images.
For example, I used a space war image meant to promote my upcoming book The Jive Hive. I have a wonderful publisher who will supply me with a cover page and graphics (I presume) for my book cover and personal use. When that arrives I will be so elongated. Until then serrate head and puzzle. Perhaps be in deep doo doo yet again for ignorance and hitchhiking the galaxy without permission.
I found a site with free images but my computer detected malware when I tried to download the image. So I saved the image in Dropbox, not ideal, it has their logo on it.