settings.py should look like this:

```python
THUMBNAILS = {
    # Other options ...

    'POST_PROCESSORS': [
        {
            'processor': 'thumbnails.post_processors.optimize',
            'optimize_png_command': 'optipng -q %(filename)s'
            'optimize_jpeg_command': 'jpegmini -q %(filename)s'
        },
        {
            'processor': 'my.own.post_processor',
            'foo': 'bar'
        }
    ]
}
```