CC Search is a simple but very effective gateway to a wide variety of search services such as Europeana, Flickr, Google Images and Wikimedia Commons (and many more).
Rather than a search engine in itself, it carefully formulates search queries and passes users straight to the respective search interfaces.
In the case of Europeana, the default options (searching for items that allow commercial use and modification) creates a query such as:
The key parts here other are the qualifiers that filter by specific licenses. In the search box you will see:
bicycle AND RIGHTS:*creative* AND NOT RIGHTS:*nc* AND NOT RIGHTS:*nd*
which queries for any matches of the search term 'bicycle' that have a creative Commons License, but not one which matches *nc* (non-commercial) and not *nd* (no derivatives). In practise this means Public Domain, CC0, CC BY and CC BY-SA.
Likewise if you untick either or both of the checkboxes on the search form those filtering elements will be removed from the query and hence give more results.
You can also install CC Search as a custom search engine in most browsers.