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I'm not saying you'll see a massive increase in upvotes.

2 points:

  1. You may be missing out on votes from manual curation teams on Steemit without souring your images. This mainly applies to "newer" accounts.
  2. If you're not using free to use images (even with sourcing) without the author's permission you could be liable for a lawsuit. (I'm not a lawyer so not sure on this, but I'm deciding to err on the side of caution)

You are correct, a lot of manual curation teams will overlook your posts if you don't properly source your images, and many steemit users will not upvote you if you have not sourced or given proper credit to the owner of the images you have used.
Also, you are correct in that using an image without the authors permission could land you in court with a very expensive lawsuit. Copyright law states that the photographer owns the copyright on any image they have taken and it applied automatically from the moment you press the shutter on you camera. You don't need to register it or add a watermark or the copyright symbol to it.

You don't need to register it or add a watermark or the copyright symbol to it.

It's a nice practice for people to do it on Steemit, but agree it's not required. Unless you talk about the picture you're posting such as look at this picture at such and such location. You could be lying, but think it helps give clarity it's your picture. Also, doing it is more for clarity that it's your picture than claiming "ownership" of the picture. Just my thoughts though.

You are absolutely correct :)