It's hard to determine whether or not a photo is public​ domain or not, especially for the average user. I always suggest to users that they put the source where they find the photo. It's great to have photos here that aren't found anywhere else, but that's an unrealistic goal for a site like this.

Some are of the opinion if it isn't labeled with usage rights then it should be considered copyrighted, and then there is the other side that believes if it isn't stated then it's "Public Domain."

In my opinion, it is a gray area that users are going to need to be tolerant on. Personally, when I'm browsing, as long as the source is cited and it isn't tagged #photography then I give it a pass.

It takes too much work to determine if a photo that appears on hundreds of sites is copyrighted or not.

It took me a while to decide to flag this article, and I'm still not sure how I feel about it. Because there are a number of users here including me that encourage original content we expect that when a user posts something that it is theirs.

I don't always cite the source of my photos, but they are either my own or from pixabay.com. It's probably something I should consider doing, even if the source is myself. But I also don't expect one of my posts to go this viral either. :)

I do hope that you consider citing the sources of your photographs in the future.