Go to http://www.pixabay.com. All images there are free to use. You can search for images, then download the one you like onto your computer. Afterwards, you can add it to your Medium article. Please try it out, if you get stuck you can just reply to this message and I’ll try to help out. There are a few other sites like Pixabay, which provide images for others to use, but I can usually find what I need on Pixabay and have rarely needed to search elsewhere.