Basically, EVERY artwork, photograph, story, sculpture, creation is the creators copyright and property. Every single one.
Then they may decide to issue it for public domain. Public domain sites are NOT always public domain sites. Oftimes they are infringing just hoping to make money from your purchasing from advertising on their pages.
- Or the artist may sell licenses for others to use their work. On which case purchase only after reading the terms of that license. All can be different.
Or the artist may be dead and copyright not continued by their estate. Again, due diligence is needed as copyrights may be extended, or owned by other parties. Just because you’re dead does not mean the copyright is on its way out.
The law states you must do due diligence to find the owner of a copyright. You therefore must prove this in a court of law if they request it. So keep a detailed log of where your images come from.
If you change an existing copyrighted piece then it still belongs to the creator, only your changes are copyrightable. If you use a filter to change the image then often you cannot copyright those changes as the filters belong to THEIR creator.
All of this is grey area as it depends on many factors.
Such as the photograph of the Mona Lisa (for instance) that you use to paint from is actually the copyright property of the photographer who took her photo. Seriously. But the proof it’s theirs would be on them.
Copyright is a convuluted area and always in flux. So my advice is to take your own photos. Paint from your own imagination or photos. Use your own devices.