Transparency of Images: Predefined or Dynamically?

I actually wondered what I should do if I need to use a partly-transparent image as a Background for a Fragment, for example.

Should I export the PNG with 50% transparency and use it in my App?

Or should I export it with 0% transparency and use android:alpha='0.5?

Are there pros and cons of each method? (Assuming I never need to change the transpancy in my app) Maybe there are performance differences?