One growing resource for images is Europeana, an aggregation service that is slowly working to build a massive database encompassing the holdings of Europe’s many “memory institutions”. Europeana conveniently allows you to filter your searches by copyright status. If I search for images with the term “landscape”, the left sidebar allows me to further refine by specific copyright status, and by the more overarching categories of “Can I use it?”
Europeana is a great resource for images