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In short: the master can be thought of as a coordinator; it does not route all traffic between nodes through itself, but helps nodes (at startup, and during execution) to find each other. Nodes then communicate directly, via either TCP or UDP (this can even be configured per subscriber).

In your case, images will go directly from the publisher to the subscriber, after the master has provided them with enough information to be able to find each other.

In short: the master can be thought of as a coordinator; it does not route all traffic between nodes through itself, but helps nodes (at startup, and during execution) to find each other. Nodes then communicate directly, via either TCP or UDP (this can even be configured per subscriber).

From the wiki/Master and wiki/Topics wiki pages.

The role of the Master is to enable individual ROS nodes to locate one another. Once these nodes have located each other they communicate with each other peer-to-peer.

In your case, images will go directly from the publisher to the subscriber, after the master has provided them with enough information to be able to find each other.