You can change images using page settings on the left panel.
Open corresponding page.
Find a button "Choose another image" there (or similar button) with recommended image size:
Select an image from Media Gallery (Media Gallery).
Other Articles
- Overview - Creating and Customizing Your Webpages
- Make Pages Public or Hidden
- Include or Remove a Page link in the Main navigation bar
- Rename a Page
- Change order of Pages
- Edit Page Content
- How do I edit or change font styles and background colors of text areas?
- Preview changes prior to Applying
- Can I add a video to my website?
- How do I change the position of captions on the Welcome Page?
- and 15 more ...