Modify Article Content type

Step 1 Go to Structure > Content Type.

Step 2 In Article row, click "manage fields" link.

Step 3 Create "Image" field with following settings:

Click save and set following properties:

Step 4 Go to "Manage display" tab > Default tab, set following properties:

Step 5 Go to "Manage display" tab > Teaser tab, set following properties:

Step 6 Click "Save" button.