Answer
Error messages are designed to help you fix invalid requests to ArcWeb Services. If you include a value that a service doesn't recognize, it will return a message that includes valid values in parentheses. Refer to your request and select one option from the list in parentheses.
An example of this type of message is:
"Invalid map image format 'JPG2'. MapImageOptions.mapImageFormat not (png, png8, gif, jpg)."
For mapImageFormat, your requested value should be 'png', 'png8', 'gif', or 'jpg'.