There is a misconception that once something is published on the Internet it becomes Public Domain, this is a myth. Copyright exists on all created content and images and consent must be obtained from the copyright holder before publishing elsewhere. It is not enough just to copy and publish with a reference to the originators location, permission must be obtained.
The reality is that with some images the copyright holder is long dead or is likely to be untraceable so republishing without permission is likely to be overlooked but it is still wrong to publish and allow the impression that it is your own work.
Read more about Copyright Law here.
All of my images are available for use, however permission must be obtained first. Charitable or personal use will normally require only an acknowledgement while commercial use may be by negotiation.
