I just found a very useful blog post on tricks to find out who owns an image and what the copyright might be on it. Although the article refers to American copyright the tips and advice offered are sound. Tracking down the origins of an image can be difficult but it is worth it.
“Twitter is a wonderful place to share, and of course we Tweet, reTweet and Favorite to share back to our Personal Learning Network [PLN]. But what if I want to use that image in a post? Do I have permission? First, as I usually do, I asked my peep if she had created the image. She didn’t know the source, which is common in Twitterverse because we like to share a good thing. But I really did like the image and wanted to know if I could use it.  Fortunately, Google provides an image search.”