_export_field
Specifies the name of the parameter which should contain the exported image information.
Default Value
Defaults to the value of the _import parameter.
If there is no _import parameter, or if the
_import parameter refers to a URL, then "file" is used.
Remarks
Use this parameter along with the _export parameter if you would like
the image information sent with a specific parameter name.
See Also
_import,
_export
Samples
Round-trip image editing using a specific HTTP GET parameter

Edit This Image And Export It Back To Me!
http://www.picnik.com/service/?_apikey=YourApiKey&_import=http%3A//www.picnik.com/graphics/api/api_sample_2.jpg&_export=http%3A//www.picnik.com/info/api/reference/_export_field&_export_agent=browser&_export_field=imageurl
This is an extension of the sample we gave for the _export parameter.
In this case, we've added the _export_field parameter to tell Picnik to
send us the image data via the imageurl parameter instead of the default file parameter.