2x images are generally images that are optimised for retina screens. They're scaled to twice the size of 1x images but since retina screens have twice the pixel ratio of most other screens they output the same size visually as 1x.
So a 25px image in width and height will look the same as a 50px width and height image in 2x.
The smart objects within our Playground templates already account for this. The smart object for a standard banner is 640x100 (320x50 @2x). We've provided a preview frame so you can see your design in context.
Please do not design at the original dimension and stretch them out to fit the template. This will result in images looking poor/blurry on a mobile device. Most Desktop/Laptop monitors are low pixel density so make sure you test your Wizard builds on a modern mobile device.
Refer to this online guide for more information.