Content that a person produces belongs to them (is under copyright). In America, it is not required that a person specifically say that they copyright their work. It is automatic. Other countries vary.
To use someone's work, without threat of legal action, requires permission. Some people will specifically waive their copyrights by saying something to that affect (i.e."Feel free to use my images...") but this is relatively rare. Others will offer their work through a Creative Commons license. The Creative Commons license will specify how an image can be used. Some ask for attribution (identifying the author of the work), non-derivation (not making any alterations), and/or non-commercial (not to be used for making money). Flickr has a nice search feature which can search only for photos that are offered under a Creative Commons license.
Where it gets tricky is the concept of "fair use." This is an exception which allows somebody to use a copyrighted work. One example is for criticism. For instance, if you are reviewing a movie, it is generally accepted as fair use to use a still image from the movie (or even possible very short clips). Another typical fair use is for news worthy events.
Another good source of photos that are available for public use is the government. For the most part (not absolutely, however) government photos are in the public domain (being produced by the government they essentially belong to the people). Several government agencies have pretty decent photo libraries available.
As a general rule of thumb, when sourcing photographs (or other works), you should try to be as specific as possible. Include an author's name (if available) and a direct link to the content. I clicked on a couple of your photos from your hubs and they just seemed to take me to the homepage of the originating website.
As for risk, it's probably not very high. Copyright issues are often pursued for monetary reasons. Either you are making money off of someone else's work or you are in someway compromising their ability to make money off of their own work. For most Hubbers neither of those will be true. That isn't a guarantee, but, for example, no one is going to go after me who gets like 50 views a day total. It just wouldn't be worth it.
Don't feel alone in your confusion. It is a very complicated topic, that's why there are lawyers who do nothing but copyright lawsuits.