While the source only mentions "Creative Commons BY" as a license the uploaders of files added to their beeldenbank need to agree with either cc-by.3.0 or cc-by-nc-3.0 and they also use the 3.0 version of the license at their other websites
to share – to copy, distribute and transmit the work
to remix – to adapt the work
Under the following conditions:
attribution – You must give appropriate credit, provide a link to the license, and indicate if changes were made. You may do so in any reasonable manner, but not in any way that suggests the licensor endorses you or your use.
share alike – If you remix, transform, or build upon the material, you must distribute your contributions under the same or compatible license as the original.