How to insert a statement (e.g. copyright) which remains visible in mapping window?

There's a "Copyright" decoration option in QGIS already.

View... Decorations... Copyright Label.

Choose text, placement position, colour. Make sure to enable the checkbox. Doesn't have text size or font choice, but the defaults look okay. Use © if you want a copyright symbol.

This is QGIS 2.4.

You could check out the Watermark plugin for QGIS

https://plugins.qgis.org/plugins/watermark/