The "Kunsthalle Basel" has an exhaustive photographic documentation if exhibitions since the 1920ies. The Photographs not only show the works, but give also impressions of the exhibitions settings etc.
Please use the fulltext search to find images concerning specific artists (e.g. enter "Beuys" into the text field)
Access: http://data.dasch.swiss/kunsthallebasel/digital
kunsthallebasel