I found the 57 images in 2015, which I believe were made as freely available to use for commercial use. However, I have tried and can't track down the original artist(s) for crediting them, or find guaranteed proof that they are available for non-commercial or commercial use.
(The 2016 version was published on the Apple store for 4 years, but it was free to download, no ads, and no in-app purchases. It is also on tomsnively.com and gamejolt.com for free. This game has never earned any revenue.)
I found the 57 images in 2015, which I believe were made as freely available to use for commercial use. However, I have tried and can't track down the original artist(s) for crediting them, or find guaranteed proof that they are available for non-commercial or commercial use.
(The 2016 version was published on the Apple store for 4 years, but it was free to download, no ads, and no in-app purchases. It is also on tomsnively.com and gamejolt.com for free. This game has never earned any revenue.)