Unfortunately the Image Upload in the HTML editor field has been removed so the user would have to upload images from the Files and Images interface and place it into the Content field with add image function and select from dropdown.

If the Image position is determined by the template and the Content field you could always add an ImageDataObjectManger from DataObjectManager Module.

in your page type class add in following

static $has_many = array (
      'GalleryImages' => 'GalleryImage'
   );

function getCMSFields() {
      $fields = parent::getCMSFields();
...

$manager = new ImageDataObjectManager(
         $this, // Controller
         'GalleryImages', // Source name
         'GalleryImage', // Source class
         'MyGalleryImage', // File name on DataObject
         array(
            'GalleryImageTitle' => 'GalleryImageTitle'
         ), // Headings
         'getCMSFields_forPopup' // Detail fields (function name or FieldSet object)
         // Filter clause
         // Sort clause
         // Join clause
      );
      $fields->addFieldToTab('Root.Content.GalleryImage',$manager);

...

      return $fields;
}

add the following dataobject in a separate file

class GalleryImage extends DataObject
{
   static $db = array (
      'GalleryImageTitle' => 'Text'
   );
   static $has_one = array (
      'MyGalleryImage' => 'Image', //relation needed for this DataObject
      'BelongToEventPage' => 'EventPage' //relation needed to point back to your pagetype, my pagetype is EventPage.
   );
   
   public function getCMSFields_forPopup()
   {
      return new FieldSet(
         new TextField('GalleryImageTitle'),
         new FileIFrameField('MyGalleryImage')
      );
   }
   
   
}

You would also need the add in following two module 'DataObjectManager', 'SWFUpload', remember to do a /dev/build?flush=1 after you upload this two module onto your server.

then in your template do following
<% if GalleryImages %>
<% control GalleryImages %>
<img src="$MyGalleryImage.URL" width="90" />
<% end_control %>
<% end_if %>

And I like it, too. Exactly what I needed!
Thank u, Ben_W!

Have a look at this page: http://doc.silverstripe.org/doku.php?id=image
The easiest way in your case would be use $MyGalleryImage.SetSize(70,70)
Please note this will add necessary white padding, ie if after resize the viewable image is 70x60, the it will add 5px white padding on both end vertically, so that your final image will be 70x70.

If you need to customise image, then have a look at the following pages:
http://doc.silverstripe.org/doku.php?id=recipes:customising_image&s=custom%20image
http://doc.silverstripe.org/doku.php?id=recipes:gallery&s=custom%20image