Copyright laws apply to images used on web pages in the same way as printed and other media. Organisations who supply photographs for use in media including websites are now routinely scanning the Internet using increasingly sophisticated software and other methods to locate possible copyright infringement. This means that YOU as a website owner, need to be sure that you are permitted to use the photographs and other images that you have on your website, if not you could be liable for some hefty fines.
So, what is copyright infringement?
To infringe copyright law, a person / organisation must have copied, or authorised the copying of either the whole or a 'substantial part' of a work, without getting the required permission (often by payment) to do so.
So what is a 'substantial' part?
A substantial part is not a specific percentage, but an important, essential or distinct part. Often the quality of what has been used is more significant that how much of it has been used. The test for 'substantial' could involve imagining whether considearble annoyance would be caused if the original work was missing the proposed material to be copied - if so, then it is 'substantial' e.g. copying the Mona Lisa's smile.
The message here is thankfully a little clearer than copyright law - make sure you know you are permitted to use all the elements that make up your website, because the whole world can see it!
Conversrely, software like CopyScape can help alert you when other people are poaching your content ...