how do you know if you are allowed to use a picture?
The law (in most countries) requires that the author of a picture always be identified on or near the photo. This is true for both amateur and professional photographers. If it is not done, it is not the responsibility of the photographer, but of the person who publishes the photo.
If it’s done properly, it is easy to find out about the copyright of a photo, directly from the author.
If there is nothing on the photo, it never means that you can use it freely. If you use it without permission, you may be liable to prosecution. You may be ordered to pay compensation to the photographer.
Publishing my pictures
There are many cases in which the use of my pictures is allowed, depending on the contracts with the organizers (pictures of certain events for example).
But I want to be clear: none of my photos are free of rights, none of my photos are in the public domain.
I have contracts and agreements with many partners and organizations, which can publish photos in different places. However, if you find photos on the internet, this never means that the use of these photos is free. The only way to find out is to ask.
In particular, the commercial use of my photos is always forbidden unless explicitly agreed. The use on pages or accounts of social networks (facebook, flickr, instagram, pinterest, tumblr, etc.) is always forbidden unless I give direct agreement.
How to use a picture properly?
If you wish to publish a photo taken by a photographer (whether amateur or professional), you must:
Obtain the photographer’s permission. He will be able to give you the conditions, and possibly the price.
Mention the name of the photographer on or near the photo. It doesn’t necessarily have to be big, it just has to be clearly legible. This is true even if you pay for the photo. Mentioning the name is not advertising for the photographer, it is simply a legal obligation in most countries.
Use the photo as it is, without cropping it, without modifying it. If you want to modify it, either its format or its colors, you have to ask the photographer. Again, this is a legal obligation.
If you find these conditions unacceptable, there is another extremely simple solution: do not use the photo.