Pinterest, sharing other people's images and copyright issues
If you follow the various tech-business and start-up blogs or the headlines at Techmeme, you probably heard of Pinterest, one of the trendy start-ups du jour. This is obviously not a start-up blog, so why are we mentioning it? Answer: because it decidedly intersects with the world of photographers.
As the name hints, Pinterest lets you "pin" pictures of "interest" you find while surfing around the internets. The pinned pictures are republished on the Pinterest website. Here is an example, someone liked this popcorn cupcake picture (buttery vs buttery bokeh) and "pinned" it to Pinterest.
This can obviously lead to unauthorized republication of copyrighted and non-public-domain works. The on-going controversy and situation so far is summarized in a detailed article at The Verge, with a robust discussion in the The Verge comments.

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