Resize images through the URL
You can resize images by passing a size parameter in the query string portion of the URL. The size parameter replaces the larger of the height or width parameters. 
EXAMPLE: http://<your_domain>/images/<library_name>/image.png?size=200
By default, this feature is turned off. To enable it, perform the following:
    - Navigate to Administration » Settings » Advanced.
 
    - In the tree, click Libraries » Images.
 
    - Select Allow unsigned dynamic resizing of images  checkbox.
 
    - Save your changes.
 
 IMPORTANT: This is a legacy functionality and it might cause performance issues.