Yeti
All original works of authorship, including pictures, are automatically copyright protected for 70 years after their creator dies.
That means any pictures you find *ARE* copyrighted, unless they very clearly unquestionably say that they are not. At most you might find something with a Creative Commons license. You are not likely to find much if anything that's truly dedicated to the public domain and "copyright free."
Nuff Sed
Photos only lose their copyright when it expires, typically 95 years after publication, in the USA (for works made for hire), and at least 50 years in other countries. Copyright on works of identified individuals may last 70 years after the death of the photographer.
Anything published in the USA prior to 1923 (other than a sound recording) is no longer copyrighted under US laws. Works of the US government are also not copyrighted in the USA, meaning there are millions of photos online that are not copyrighted. It's your job to determine which are those and which are still copyrighted, depending upon how you intend to use or distribute copies of someone else's works.
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No viruses from pinterest. The images are all copyrighted and WILL get removed. Most don't though.