You can copy, modify, distribute, and use the images, even for commercial purposes, all without asking for permission or giving credits to the artist.
I never think copyright is much of an issue anyway when using photographs for paintings, especially as in my case, you never sell them. Whenever I’ve used “Internet-found” photographs I’ve usually based my paintings on several images and grabbed bits which I like where I can find them. The SAA support my view by stating on their website
You won’t infringe copyright if you use someone else’s work for private research or study – so you can copy a photograph, a picture from a book, or a painting by another artist, whether you’re working at home, at your local art dub or visiting a gallery. However, if you decide to exhibit, publish or sell that painting, you are then benefiting financially, so would need to seek permission from the artist or photographer who owns the copyright. If you want to copy a famous work of art or sculpture, you won’t infringe copyright if the artist who did the original has been dead for more than 70 years”
Anyway, here is a painting I based on a Pixabay image – its Ivar’s fish restaurant in Seattle (I am grateful to @helloheidilh on Instagram for identifying it as the Pixabay image didn’t reference the exact location.