To add an image onto a page, you will use the Image Manager from the Design editor on Event Descriptions (when editing events), CMS pages, News articles, etc. The Image Manager / Document Library is available only for users on Premium or Ultimate plans. For users on Basic or Standard plans.
From the Settings & Options > Settings > Theme & Images:
A new window will appear which is the Image Manager. Click on the folder Uploads and click the upload button to upload an image.