What is 2x resolution? [RESOLVED]

I am trying to make a game that runs on both computer and iOS, but when I import frames for an actor, there's a little message on the bottom that says:
Import an image at 2x resolution (for Retina Display.)
What does this mean? I don't want to get this wrong. Thanks.

2x resolution means that the graphics should be double what they would be in the standard (480x320) resolution. So a 2x background would be 960x480. Stencyl will resize and scale the image for you to make the game look nice. Another example is if you want a 50x50 box in your game. You would want to design and import the box at 100x100. Hope this helps!

PS: When designing and coding your game, everything is in 1x scale.

So anything I will import into Stencyl when designing an iOS game will be halved?
EDIT: For example, if I import a 50px by 50px actor, will it show up in the web version, Stencyl, and iOS version of the game as 25px by 25px?

What happens is, if you import your art at '2x', Stencyl will create a half resolution copy of it. If you import it at '1x', Stencyl will create a *double* resolution copy of it. If you check the images in our /resources directory you can see what it's doing.

I see what's going on. I import a photo and a 2x copy is created. What if I am importing a 2x photo and I want Stencyl to recognize it as a 2x photo so it can create a 1x copy rather than a 2x one?

Then you choose '2x' from the dropdown when you are importing the frames.

Thanks! 

Is this dropdown menu only available in the iOS version, because I dont' see it?