Hi,
UK law generally accepts that photographs of paintings attract copyright in their own right, due to the skill and effort of the photographer (this is in contrast to the
USA,
France and
Switzerland, where there have been landmark cases which found that such photographs lacked any 'original' input by the photographer and therefore were not subject to copyright).
So you need to try and find out who is the photographer of any work of art and either negotiate a licence to use his work, or ascertain if the photograph has been effectively made available for free via a scheme like
Creative Commons. There are certainly many of the more famous works of art which are available for free, so it may be best to try and locate them. Take a look
here for how Wikipedia approach the issue, but bear in mind they operate under US law so not everything in that article will accord with UK law. Don't be tempted to use any images from the major picture libraries such as Getty without paying for a licence as they do go after people who alledgedly infringe their copyright. Also note that some Creative Commons licences do not permit any commercial use of works (these are marked NC or use the $ sign crossed out as below:
