What size should TabBar images be?
According to the Apple Human Interface Guidelines:
@1x : about 25 x 25 (max: 48 x 32)
@2x : about 50 x 50 (max: 96 x 64)
@3x : about 75 x 75 (max: 144 x 96)
30x30 is points, which means 30px @1x, 60px @2x, not somewhere in-between. Also, it's not a great idea to embed the title of the tab into the image—you're going to have pretty poor accessibility and localization results like that.
According to the latest Apple Human Interface Guidelines:
In portrait orientation, tab bar icons appear above tab titles. In landscape orientation, the icons and titles appear side-by-side. Depending on the device and orientation, the system displays either a regular or compact tab bar. Your app should include custom tab bar icons for both sizes.
I suggest you to use the above link to understand the full concept. Because apple update it's document in regular interval