Free pictures, as in free of rights, but you might have to pay for some.
When you write articles, it is a good idea to add images. You will find a limited choice on openclipart.org.
If you publish regularly, it might be interesting to subscribe to a payed photo library (like ShutterStock or Getty), but if you seldom publish, it will be more interesting to use images for no money.
For photos, Cédric lists 43 sites of photos free of rights but you might have to pay a feee for some.
Happy images hunting !