@2x is an HD skin element. it's file size it normally doubled.
so hitcircle.png is say, 128x128
hitcircle@2x.png can be up to 256x256
though HD elements only work if the osu! resolution is high enough
so hitcircle.png is say, 128x128
hitcircle@2x.png can be up to 256x256
though HD elements only work if the osu! resolution is high enough