As a freelance writer (and indeed in any professional walk of life) you must be aware of copyright. In fact this is something we should even be aware of when helping our kids do their homework! I am not a lawyer and this is not legal advice; it is a couple of rules of thumb but in all situations seek legal advice if there is any doubt.
Take care, in particular, with two areas – text and images; of these I find that it is images that cause most problems.
The reason there are so many mash-ups appearing is that once an author has been dead for 70 years (UK law, this will vary) then the text is out of copyright. There are exceptions such as Peter Pan.
Anything else on the web is copyright the author even if they don’t say so (though it is best to – note to self, check copyright on this site!).
Don’t use images without permission. Company logos may also be trade marks which gives them further protection. You can’t (except under something called fair use) just lift a picture from the web and re-use it. Where you might blog about a book using its cover you may still want to make it clear who owns the copyright.
Photographers own copyright unless their contract transfers it to their employer. If you photograph a celebrity you own the copyright not them!
The 70 year rule applies (though didn’t at one point) unless items are crown copyright. Generally if you don’t know who took a picture the rule is 70 years from known date of printing. This means old postcards (e.g. pre 1930s) are fine, for example.
So far nothing you couldn’t find out an any of 100s of blogs. What I have found, though, is not only are there some sites with good quality royalty free photography around (I use Morguefile for example). You also get surprising results just be asking people.
Experience is of course mixed. I was recently hunting some picture of Brighton Sea Front and have found lots of people willing to sell me old Post Cards. There are also some sites for professional organisations that have a ‘contact if you want to use’ section. Take it from me they don’t always reply. I found a small website that has some astounding scans of old material – the webmaster never answers my email!
In contrast, I was researching something else, found a site that had some great pictures from the 1950s, contacted the owner who was only too delighted to send me some high-resolution scans to use.
My advice – don’t ask, don’t get!