To add an image to a page, select the Add Media button:
Select the image you want from the gallery page and select Insert into page at the bottom right:
Once the image has been added to the page, select on the image again and click the Edit button:
Under Advanced Options > Image CSS Class, add the following img-fluid. This ensures your image is responsive on all screen sizes.
You can also add on-click links, change the sizing and add alt text and captions from the Image details page. Once completed, click Update at the bottom right