Here is one of my internet pet peeves.
Frequently, people upload images they'd like others to be able to use for free. Great!
But then, they label their generous gift to the world "Royalty Free -- please take." Ug. That's not what Royalty Free means! Copyright and image licensing are complex enough without this mixup further stirring the soup. So:
Royalty Free does not mean Free.
Don't let the Free mislead you. Royalty Free (hereafter, RF) means that after the initial permission is secured, usually through money, additional uses can be made without payment. RF is multiple use free of royalties. Pay once, put the photo on a t-shirt, upload it to your website, print out some fliers, get it tattooed on your forehead. It's all clear! But you still had pay for it at the outset.
Conversely, if you are looking for free pictures and stumble on a site with images offered "Royalty Free", RF does not mean you can take them!
Contrasted to RF is Rights-Managed (RM) use, where the creator and the user reach a more tightly controlled agreement regarding multiple uses. For example, a RM image appearing in the first edition of a textbook might need to be re-licensed for a fee in order to be cleared to appear in the second edition. RM is the traditional way for licensing intellectual property, and remains in use partly because the details of an explicit license contract lessens risk and makes legal departments happy.
Anyway, enough about contracts. What if you just want people to use your pictures for free? Instead of confusing everyone with your Free Royalty Free photos, I recommend perusing Creative Commons licenses. Tag your images with a CC-license whose terms you like. Or, if you are feeling especially generous, perhaps you ought just release your images into the public domain. Sounds scary, but that's what NASA does.