image_proc needs one camera_info for every image !?

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While true, on the driver side you don't know when the application will start listening on the messages so you should just constantly stream them. On the application side, you don't know if any of the camera parameters will change during runtime (e.g. binning or ROI) so you should listen in on them. If you want to save CPU and memory and you know your parameters are not changing during run time you could run a custom version of the image_proc nodelets and do two things:

1. Edit the required nodelets to unsubscribe to the topic once the camera info has been received (see here https://github.com/ros-perception/ima... )
2. Publish your camera info message only if there are subscribers detected.