Multiple subscribers to service

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I may have misunderstood the concept of ROS services, but is it possible to have multiple client calling for the same service?

Yes, this is possible. A service-server offers a service, that any node can use. Each client sends some information to the service and gets a corresponding response. See also here for a simple example.

For instance, I have a service running on a server that processes raw images and returns informations about them. Can I have a service that use those informations and another that register the output of server?

So, if I understand correctly, you want to send some raw images to the service server and get information about them as a response? Yes, this is possible with a service server. If your server just processes the images it gets from an external non-ROS source and you want to use them in another ROS node, then it would be better to publish them as a topic.