TabBarItems and setting their image sizes?
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.
Another option is to create a vector image PDF. What you do is resize your vector image to the smallest size (22x22) and export it to a PDF file. You can do this with Adobe Illustrator, Sketch and many other vector image editing tools.
Then you add the PDF file to your assets folder and in the Properties Inspector set "Scale Factors" to "Single Vector". That should do it!
Pretty easy to do this with ios8. In Storyboard, click on each tab item (the individual controller for the tab you want to change the image of.
Set the insets accordingly...
There is this issue though. Fixed with this.