If you are using 1366x768 you (or anyone else) will need to force this option to work. (resolution height is lower than 1000px)
Find in main osu! folder, file named "osu!.<username>.cfg" and open it with notepad.
Then you need to find this line :
HighResolution = 0
And change 0 to 1.
If you want to make skin for every resolution just make 2 backgrounds : Normal and HD (this is even recommended for everyone if someone is making HD skin)
Also, remember that @x2 MUST be 2x size of normal sprite since it will be way to small.
Example :
"pause-overlay.png" - is normal pause screen. Size : 1366x768px
"pause-overlay@x2.png" - will be replaced if person is using resolution with height above 1000px or forced it (this thing above). Size : 2732x1536px