Navigate to the page you wish to modify

Click on Edit.

Click on the  sign below the title box.  This prompt will also appear between any existing web parts. 

From the web part gallery select Image.

An image selection window will appear. From the left hand navigation bar select a source from which to add your image.

Selecting Stock Image will allow you to search and select an image from Microsofts default gallery.

Selecting Web Search allows you to search the internet for images using the Bing search engine.

Selecting OneDrive or Site allows you to search images stored in your OneDrive or current site.

Selecting Upload will allow you to add an image from your local device.

When you have selected your image click Insert.

If you add the wrong image in error you can elect to Discard Changes and the page will revert to its pre-edited form.

To make your new image visible to colleagues you will need to Republish your changes.