Canva is a FREE graphic-design tool website.  It uses a drag-and-drop format and provides access to over a million photographs, graphics, and fonts. The tools can be used for both web and print media design and graphics. This is perfect for creating your Web Graphics and eBook Covers, should you want to re-brand them.

Lunapic is a FREE online photo editing processor. You can upload images from your computers or link image URLs to add effects, crop, and more. LunaPic let’s you to upload, edit, and share your pictures. It lets you get really creative with your online images, graphics and photos.

All contents in Pixabay are FREE and released under Creative Commons CC0, which makes them safe to use without asking for permission or without giving credit to the artist/s. You can use them for personal or clients’ projects or even for commercial purposes. – Source. 

Unsplash is a website dedicated to sharing copyright-free and HD photography under the Unsplash license.  Beautiful, FREE images and photos that you can download and use for any Project or Digital products, eBook Contents or eCovers.