When you create a course on Eduflow it will automatically get a random course thumbnail. Such as this one:
To change the course thumbnail, click the context menu and select "Edit thumbnail photo".
In the new menu you can shuffle between a set of random images or link to something yourself.
If you don't know how to link to a picture, follow these steps:
Go to (for example) Imgur
Upload your picture
Right click on your image, and press Copy Image Address
Go back to Eduflow, paste the link you copied in Edit thumbnail photo
Save your changes