Hardcoded image field name

Method value_from_datadict(self, data, files, name) in widgets.py has hardcoded image field name:

``` python
uploaded_file = UploadedFile.objects.get(id=data.get('image'))
```

It works with example but when you specify other name like:

``` python
widgets = {
    'picture': CicuUploderInput(options=cicuOptions)
}
```

it breaks.
