I´d like to ask in generell if there is a way to store Metadata (e.g. Copyrightinformation) regarding uploaded images directly from within JCE? maby with a plugin, etc.?

It may be possible using a plugin, that writes the metadata to the image after it has been uploaded, but it doesn't look like a simple thing to, and requires a special PHP library and a little bit of research.

If I have time to work on this, I'll let you know if it is successful.