Prompted, as I mentioned yesterday by a post from Andrew 'Loki' Saunders in the Bloggers Wargames Group on Google+. I suggest you read it, and the comments: I'm deliberately not paraphrasing the contents here, as I'd like to ensure any search queries on the matter get directed to his blog.
I am not a lawyer, so please don't consider this post legal advice, or sue me if, by following your interpretation of my interpretation, someone else sues you :D Your country's copyright law may be subtly different to mine.
That said, let's talk about both your copyright and other people's :)
Copyright, in general, subsists in any expression of an original work. You can't copyright an idea, but you can copyright any expression of that idea in any material form including digital, at which point you own all the rights to it, unless and until they expire or you choose to give some or all of them away. This therefore means, technically, that anything on the Internet is copyright its original author, and you may not copy it (beyond the necessary magical copying that happens behind the scenes to allow you to view it) without their permission.
Let me just repeat that one. Just because it's on the Internet and you can see it, it doesn't mean you can copy it and use it, unless the author explicitly or implicitly (by a blanket statement) gives you permission.
Regular readers may notice the recent appearance of a Copyright page on this blog. It's a Creative Commons licence, in fact, which says, in a nutshell, you are free to use or modify my work for any non-commercial purpose provided you credit me. All other rights are mine, but note that I may choose to grant you (for example) commercial rights if you ask and we reach an agreement.
It's dead simple to set up a page like that one - in fact, the Creative Commons folks provide you with copious links and sample wording, and it costs you nothing. We are a friendly community, and it's simple courtesy to respect someone's rights (and hey, they get a link from you, and will probably return the favour, so we all win). Make cool stuff, share it, give credit where it's due. Simples!
Other things you should be aware of:
- Amazon book cover images used for the purpose of driving traffic to Amazon and made by their Link To This Product tool implicitly have permission. I would suspect that technically they don't for other uses.
- Flickr, Google Images and Wikipedia, among others, make it easy to find out the copyright/use status of an image, and what you need to do/say in order to be able to use it. It's not hard to add a caption in Blogger with the appropriate stuff.
- A lot of national archive photos from many countries are free to use.
- A photo of a copyright work for review/illustration/discussion purposes is covered under fair use.
- Straight digital copying of art from a copyright work for review etc probably isn't - it's certainly a grey area, and I have a couple of images on this blog I need to get OKs for :D
- (In fact, I have a few pictures generally I need to check out!)
- Some material is genuinely out of copyright, and fair game. Note that the typical period is 70 years, which takes us back to 1943, usefully!
But, once again...
- Just 'cause it's online, doesn't mean it's free!