add sizes to image placeholders

@benmai @aquilax @kenkouot 
This is just a start of work to pass width and height to image placeholders
so we can style them correctly before they load

I started this on hackathon and will resume the work when I'll have time
This could be:

```php
$width = isset( $config[ 'imageMaxWidth' ] ) ? min( $config[ 'imageMaxWidth' ], $width ) : $width;
```