That's a good experience you had, there is a great team and community around phaser, that is why I like it too.
The graphics were developed by a very talented artist maungler whom I developed the game with, he will do a breakdown on how he developed the art and the tools he used.
Sometimes it is a pain centering or placing game items on the canvas but depending on your use case there are some ways to bear the pain.
The origin of a game object determines how it will be positioned. X coordinate increases as you move to the left, Y coordinate increases as you move to the bottom.
There are also inbuilt methods for positioning game objects depending on your use case
See the following phaser labs