Click on Node overview in the Content menu.
Edit the page you want to add an image to by clicking the Pencil button.
Depending on your sites setup, add the image as Featured image or Additional media. In this example, we'll use the former. Click the Open folder button under Featured image.
Now upload a new image or choose an existing image. In case you want to upload a new image, click the Upload file button and choose an image from your computer, or drag & drop a file in this popup window.
In the popup we have the chance to categorize the newly added image, do this by clicking the image, then click the change type dropdown and then click Photo's (the name might be different or unavailable in your CMS). Finally click the uploaded image again and click Choose to add the image to the page.