I am using Geoserver to deploy a WMTS service with the help of an already existing WMS service. In general it works out well but i need to add some "terms of use". Is there a way to add something like that to the metadata of the tiled images (png) or to watermark the tiles?
WMTS doesn't really have the concept of metadata associated with tiles, the best you could do is a note in the layer abstract which would appear in the capabilities document (and is unlikely to be read by a human).
One possibility for watermarking is create a polygon layer that is larger than your existing WMS layer, then create a graphic fill for it using your watermark using a graphic-margin to spread it out if necessary. Then combine the two layers in a layer group so that it is suitably watermarked in the tiles. Make sure to mark the WMS layer as not Advertised to prevent people requesting it without the marks.