I can see a few different solutions to this situation, but I am not quite clear on the implementation you have in mind. Is the image being generated and then stored somewhere on the server, waiting for the user to request it, or is the image to be generated dynamically at the time the user requests it?

In either case, honestly I think the actual image would exist outside the web service as a distinct entity on the server, either as a static file or an image generation program. The web service would simply give the client a link to request the image. The server would then deliver the static image or generate and deliver a live image.

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