Here’s an awesome list of free image sites and image tools:
http://thenuschool.com/free-stock-photos-websites-awesome-list/
I particularly enjoyed playing with the Pablo tool (https://buffer.com/pablo). It’s an online tool by Buffer that allows you to add text to images and post to social media. Easy to use.
Another cool tool : Social Image Resizer Tool . The landing page includes info on best sizes for social media type. Great info.