unfortunately i don't think it's possible to remove white backgrounds on images with coding alone D: at most you can lower the opacity of the treasure div so the images appear more faded, like this
{opacity: 0.5;}
hopefully that works somewhat? ; v ;
while you can't change the images themselves, you can fake a transparency effect by reducing the opacity of the treasure items, then adding darker boxes behind each item, so that the translucent white + the dark color = the overall background color. it takes a lot of fiddling with positions and color and is overall a bit of a pain, but i could try to guide you through it if you want to do that.