How to Sharpen Images for iPhone Usage

I have some images that i want to use on my iphone application. The problem is i think that the resolution is not right cause its not sharp at all when see it on the device. This is how it looks when i zoom in a bit on it:

enter image description here

The idea is to use this image as an image for a UITabBarItem.

Any help and general advice on how to make it sharper & better etc.. or how to make images for iPhone devices and make @2x, @3x etc??

Thank you

Solution

The tab bar icon size will be displayed about 25 x 25 point. (maximum: 48 x 32)

What does it mean?

Assume, you would like to use an watch.pngas the tabBar icon. So you need three icon to achieve it:

watch.png -> 25 x 25 pixel (for iPhone without retina display)
[email protected] -> 50 x 50 pixel ((maximum: 96 x 64 - for iPhone 6, 5, 5s)
[email protected] -> 75 x 75 pixel (maximum: 144 x 96 - for iPhone 6 Plus)

For more details see [Human Interface Design]

Of course, you need a vector graphic image to achieve it and export it in the 3 size. A designer should know the details. Or you can buy some icons (/download some free icons) in your topic. Several flat icon set are available, e.g.: