I use PicsArt, the App is free with an occasional ad. Pictures and images free of copyright from Pixabay, there are other sites, can be cropped and played with in the App, but most important, you can add text for title and author in different fonts and colours. You can also use any online resize file App if you need to get the image to fit the book cover without cropping.
If you want something closer to a program like photoshop there is a couple of free no download site I can recommend.
The first is pixlar the second is photopea both findable via google or other search engines.
Royalty free images are best found the same way via an online search rather then an application that ties you to a small selection of images.
Cutting up and collage related images can also be a way to void any potential issues.
As this makes the "image copyright" somewhat tentative and whilst I cannot assure full indemnity it should be fairly safe.
Last but not least ..You could post of one if the threads that offer the service free here.
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