If someone shared a cool photo with you on Facebook or Pinterst, you might want to write a blog post about it.
The post will be a lot more interesting if you can actually include the image. But you're not allowed to put copyright material onto your blog without permission - and how do you know if the picture that was shared with your is copyright or not, and who owns it.
Google Image Search may be part of the answer. Find out about it.
It won't tell you if a picture is definitely avaiable or not - but it gives you information from you can make some judgements based on when it first appeared, and what sort of web-sites it has appeared on..
Remember - although you're not allowed to publish copyright material, you will only get "caught" if a copyright owner spots something on your blog and takes action by filing a DMCA report. There are a great many items which are fine to share, even though they're not technically in the public domain, because no one is actively claiming ownership - and image search gives a way of validating ownership claims made by chancers.
