A few days ago a user in a Facebook group I follow asked other members if they had reference photographs that she could use in her artworks. Many members kindly uploaded their own photos for her to use, therefore giving their permission to reproduce them. However, one member surprisingly advised her to "just look through Google images", which I would strongly recommend you not to do. Whenever a photographer, either amateur or professional takes a photograph, the copyright is theirs. The copyright remains theirs, unless they specifically and clearly state otherwise. Whether or not the photographer uploads their photo to a public site such as Instagram or Facebook etc. is irrelevant, the copyright belongs to them. Incidentally, this is the same for your artworks, the Artist automatically maintains copyright even if a painting is sold, unless otherwise stated. You may think that it is "not such a big deal", if you are "only using them to practice"