Ionic 4 ion-img transparent not work

Hi I have a problem with a background image.
I would like to put a background image to the header.
This is my code:

  <ion-img cache="true" src="/assets/header.svg"></ion-img>
    <ion-buttons slot="start">

The red line is the Header background, and same to work, but may ion-img tag is not transparent. I’ve try with png but not work.I ve try to set background transparent via css but not work.
If i set ion-img to background red works but not with transparent:

the img with transparent:

the img with red: