Publicising your work on social media - Twitter, Facebook or whatever - can be equivalent to leaving a banknote stapled to a park bench. It may not be legal for anyone to take it and use it. But... you know what's going to happen.
Consider for example the case of Haitian photographer Daniel Morel. He posted pictures of the earthquake that devastated Port au Prince on 12 January 2010. He uploaded them to to Twitpic, a third-party application of Twitter. One Lisandro Suero then copied the photographs onto his own Twitpic page and Tweeted that he had "exclusive photographs of the catastrophe for credit and copyright". Agence France Presse (AFP) transmitted them to Getty Images credited to Suero. They were used on front pages worldwide.
Morel sued. AFP counter-sued for "commercial defamation" and "an antagonistic assertion of rights". The defendants also claimed that Twitpic's terms and conditions at the time gave everyone in the world the right to re-sell images posted to it.
Morel was awarded $1.2 million in 2014, but - leaving out a lot of nasty detail - we're still trying to find out what, if anything, he received.
Our advice? If you must promote your work this way, take the trouble to make versions of your images to upload that are as small and degraded as is consistent with this.
Consider "watermarking" them with "© Jo Schmo" visibly across the middle, but be aware that there have been cases involving publishers removing such labels. Do not proceed without first checking the terms and conditions of the service you use.
Text © Mike Holderness & previous contributors; Moral rights asserted. The collection (database right) © National Union of Journalists. Comments to firstname.lastname@example.org please. You may find the glossary helpful.
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