Add image field or other fields from node to a custom Block in Drupal 8
Use Views module for this. In D8 the views module is part of the core, in D7 you need to install it  

1) Go to Structure > view  

2) Add a view which shows content of type Basic page. Check on create a block.

3) Select the fields to be shown as image (as you want) 

4) Under Advanced (on the right side of the view, click on it to expand)

CONTEXTUAL FILTERS click add, and select ID (D8) or Nid (D7) and then choose "Provide Default Value" and Under type select "Content ID From URL". click on Apply and save the view. 



5) In structure >blocks, place the block in the region you want to on the content type you wish (In your case Basic Page). 

6) Go to your basic page content  and you will be able to see the image which belongs to that node only in the view block you have placed.