How to upload a file in Drupal 8?
"Content entity" types like nodes (content) are field-able, so you can choose what data are attached to them.

The "basic page" content type only comes with a body field out of the box, but it's a cinch to add new ones:


Visit /admin/structure/types
Click "Manage fields" for the "basic page" type
You'll see a list of the fields currently attached to the content type. Click "Add field"
Select a field type; "Image" would probably make most sense for your needs
Follow the rest of the wizard and choose suitable values (should all make sense)


When you visit the node add/edit page for a "basic page", you'll see the new field, and you'll be able to upload your images.

Next you'll probably want to visit the "Manage display" tab to choose how the images are rendered, and the "Manage form display" tab to choose which widget is used.