The license that protects the product is CC BY 4.0, so you can use any of the images as long as the requirements are met, namely:
1) Mention the authorship properly, both in the content of the game and in the page where it can be downloaded or purchased.
2) Mention, in a similar way, that the images are protected with CC BY 4.0 (and put a link to it).
3) Indicate if the images have undergone changes or are the original ones.
A possible text that you could use to fulfill all the above at the same time and use it for your game is the following:
"The images used in this game are from Skeleton by José Córdoba (Zeta) and are protected under the Creative Commons Attribution 4.0 International License (CC BY 4.0). These images have [indicate whether they were modified or left in their original state]".
Of course, needless to say, I would love to meet the game if it is finally published.