for webnovel, the pixel size for cover is 600 x 800. use pixlr sites or befunky. they are good for photo editing and resizing. when you open pixlr or befunky, you will find photo resize option in crop/resize. add 600 for width and 800 for height.
Its better to not use other artist photos or photos with copyright coz you might not wanna get into legal problems in future. there are many sites like upsplash, pixabay...finding good photos there is a pain. You will find some photos and just mix them together in pixlr/editor. check sites for how to add layers or photo editing.
other option is google image search. in tools, go for option : image for non commercial use with modification. search for images with key words. you might want to try png format image because they are easier to edit. thats all I know about photo editing and book images. pixlr editor is also good for photo editing..then, there is photoshop and canva. online apps are without limits.