On Chrome, I opened up the game, right clicked a tile that had an image, and at the bottom of Chrome's right click menu there is a listing that says 'Inspect Element'. From there I just examined the css that modified the title. The tiles are probably dynamically given images via Javascript/jQuery, but the css looks like this when a tile has an image:
Code:
.tile-1 .tile-inner {
background-image: url(http://img2.wikia.nocookie.net/__cb20130710011141/flightrising/images/3/38/Egg_Nature.png) !important;
}
So, that would also be .tile-2 .tile-inner, .tile-3 .tile-inner, so on and so forth. Again, this is not done by editting a css file directly, but by Javascript telling it to apply a background image and remove it when you can't see a tile, etc. You'd have to see what they're doing in the Javascript.
You'd be better off finding a different free source version of 2048 that already has images. (
Also this??? I think this was used to build the FR version with images. )