Here are some sites that offer free images, but use at your own risk since right holders can always change their mind.
Here is a source for 10 sites with rights free images.
Find Creative Commons (free) photos:
Other helpful tools (good for cover mockups):
Need an icon? Check out:
Keep in mind PubWriter does support AwesomeFonts and all the Bootstrap fonts.
If I am just looking for random safe images to use, I usually start my search here. You can't just grab images you find off Google images, you don't have the rights to use those images and there are companies who make a living by fining people who use them without permission.