(Or, the non-photographer's guide to image use) It's a truth universally acknowledged that articles, newsletters, blog posts, posters, and basically anything involving blocks of text can be improved upon by the addition of an image. When you're writing about the local cycling club's criterium or producing a short introduction to crochet and macrame, you'll probably want some pictures to illustrate events or to explain techniques alongside your race report or detailed how-to. Can you take a look at the site of a local photographer and use some of his images from the cycle race? Can you conduct a Google Image Search for 'crochet' and download some photos of great examples of people's work?
The short answer is always 'No'. Just because someone has posted an image on the Intergoogles, it doesn't mean that it is free for other people to use. You can't use the china in John Lewis' window display without paying for it first, and a photo on Flickr is just the same. Images belong to the people who create them—or in some circumstances, to their employers—so they get to decide how and when they can be used and what the appropriate fee for using them is. We put them on our websites or on photosharing sites because we're proud of our work and we like to display our capabilities, but it's not an open invitation to filch them.
There are a few exceptions to this rule, but until you know better, work under the assumption that every photographer keeps the tightest control over the use of all of her or his images. Being confronted by an angry photographer wielding an invoice for unauthorised image use is not a pleasant situation, so remember: You can't use other people's photos. Mmm'kay?
For completeness, what are these exceptions you talk of?
Some people are happy to licence their images under Creative Commons terms. Creative Commons licences aren't designed as an alternative to traditional copyright, but a complement. They're easy-to-use copyright licenses that allow you permission to use a photographer's images under terms decided by the photographer. One photographer might let you modify and use his images commercially, but another might say that her images must be attributed, cannot be used commercially, and aren't to be modified. However, not all photographers use Creative Commons terms (you'll see it close to the photo if they do) and if there's no evidence of a Creative Commons licence, assume that you can't use the image.
If you receive an image in a press release or it's made available to you from the press section of a website, this will be free to use in the context of the product or situation. For example, when Olympus releases a new camera, it will make a bundle of images illustrating it available to me. Provided that I'm writing about that camera, I'm free to use them in an article. The National Portrait Gallery will supply a selection of images from each of its exhibitions so that if you're reviewing it or publicising one, you have photos to illustrate the article. But, you can only use those photos in relation to the relevant exhibition and they must be attributed under the terms set out by the NPG.
Some news agencies, for example AFP, are happy for you to use their photographers' images non-commercially and for personal use provided that you credit the photographer and agency and link back to the site. But, some agencies aren't. And you wouldn't want to face the wrath of AP or Reuters. Again, unless you're absolutely certain, assume that an image isn't free to use.
But what if you see an image and want to use it? What should you do?
Get in touch with the photographer! Most of us make it easy to send an email: do just that. We don't bite. Mostly. Tell us who you are, why and how you'd like to use a photograph that we've taken, and ask if you can come to an arrangement. The worst that we can say is 'No'.
That's not so hard, is it?