You should prepare 3 images icons for each tab bar item (1x, 2x and 3x).
First create the 3x at 75w 75h pixels (maximum: 144 x 96) and save it as iconTab0@3x.png.
Then resize it to 50w 50h pixels (maximum: 96 x 64) and save it as iconTab0@2x.png.
Finally resize it to 25w 25h pixels (maximum: 48 x 32) and save it as iconTab0.png.
Now all you need is to select those 3 images at your finder and drag them to your image assets.
Human Interface Guidelines
