The warning came after two unidentified subscribers to EvangelismUK reported receiving invoices from Getty Images for £900 and £1,500 after unknowingly using the pictures without permission on their websites.
Ownership information encoded in digital photos allows owners to track where their images are being used and control copyright infringement.
More than other photo stockists, Getty Images has acquired a fearsome reputation for its crackdown on copyright infringement, hitting web masters with invoices for large sums of money over improper image use.
Web masters may upload images to their sites unaware of the legalities of copyright or because the images were found without proper labelling on other websites, including websites claiming to stock only royalty-free images.
And when it comes to chasing down infringers, size does not matter. Letters have been sent out by Getty to web masters for the illegal use of images as small as a thumbnail.
Jim Currin, of Churches Together in England said: "Can I pass on the warning for all web managers to make sure all images are copyright covered or replaced with your own."