Food for thought. Thanks for everyone's comments and advice. I now know several of my paintings are illegal. Not only that, many people I know are painting illegal pictures.
Reading the Wikimedia Commons article - thanks for the link - it seems that copyright rules on "derivative" works are quite strict. I was amazed to read that taking a photo of a child holding a Winnie the Pooh toy could be considered a derivative work and an infringement of Disney's copyright.
I realise the article was referring to US law, but no doubt English law is very similar. The important thing is how the law is interpreted in an an individual case. It must be quite rare for painters to be sued for using a photo without permission, but it's not really worth taking the risk.
A library of copyright-free photos would be great. There are some
here.
David