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Not sure if I understand your scenario correctly, but you want to 1) subscribe a topic 2) process the topic content 3) republish the modified topic.

Depending on what process you want to add to the topic, you may find topic_tools suitable for your need particularly transform command.

Since image_transport republish provides a way to subscribe to a topic that's being published and republish it, without adding custom processing, that might not help your case.