the CC-BY-SA 3.0 license txt leads me to believe, that you are free to publicly display the LPC artwork images in an app, as long as you are able to credit such artwork as being CC-BY-SA 3.0, include a URL to the CC-BY-SA 3.0 license, and you give credit to the original artists, and mention the affiliated LPC, and you provide a URL to the LPC's sites, and a URL to Opengameart where they can obtain said images.
You also can create derivative works, liked recoloring, redrawing, changing size or other ways of modifying the images, which you can also publicly display in your app, if you label appropriately which works were changed such as "Main Character based on original Goblin by ___", and you make any derivative artwork available under the same CC-BY-SA license, and prove a url to where the user could obtain said deriviative images.
That should satisfy all of the CC-BY-SA terms.
But if youve already submitted a game for the LPC, or your modifying the existing source code on one of those games, your game is stuck under a GNU license and would required releasing your game's source code.
Assuming your using LPC artwork in a new game;
the CC-BY-SA 3.0 license txt leads me to believe, that you are free to publicly display the LPC artwork images in an app, as long as you are able to credit such artwork as being CC-BY-SA 3.0, include a URL to the CC-BY-SA 3.0 license, and you give credit to the original artists, and mention the affiliated LPC, and you provide a URL to the LPC's sites, and a URL to Opengameart where they can obtain said images.
You also can create derivative works, liked recoloring, redrawing, changing size or other ways of modifying the images, which you can also publicly display in your app, if you label appropriately which works were changed such as "Main Character based on original Goblin by ___", and you make any derivative artwork available under the same CC-BY-SA license, and prove a url to where the user could obtain said deriviative images.
That should satisfy all of the CC-BY-SA terms.
But if youve already submitted a game for the LPC, or your modifying the existing source code on one of those games, your game is stuck under a GNU license and would required releasing your game's source code.