RESTful services and

I have just read Resource-Oriented Architecture: The Rest of REST. 开发者_StackOverflow社区The reasoning behind content negotiation is compelling, but there's one thing I sometimes need, which seems to be impossible in this schema.

Let assume I've got a web service to deliver some graphs. I want users to choose between different styles of these graphs (a fancy color one, B&W, ...), but all of them are always png images. For all of them the mimetype will simply be image/png. So what is the preferred way to negotiate custom parameters?

Use querystring parameters.