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Use these instructions to create an e-book using the online service, PressBooks. PressBooks is free unless you want to remove watermarks from the e-book. The advantage is that PressBooks comes with e-book templates to give you a good e-book design.
Use these instructions if you want to more thoroughly edit an e-book using Calibre or create one from scratch.
Where to Find Copyright-Safe Images
You might want to add visuals/images to your e-book. Images are protected by copyright, so unless you have permission, you can't use other creators' works. However, maybe creators give their work Creative Commons licenses and make them available for different sorts of uses.
The sites below have Creative Commons licensed works for you to search through. Pay attention to what sort of uses are allowed. For example, sometimes only non-commercial uses are allowed and other times commercial is included.