When using no dialogue background panel, the standard semi-transparent black background gets used which looks just fine:

Now, when I want to use a unique background as "PANEL" like the following one...

...I get the following result:

It doesn't matter how large the image is, it always begins with a padding from the left and gets resized. The main issue here is that the text is wider than the background picture and I can't do anything about this.
So either the text should have the same padding from the borders or the image should use the same algorithm as the background color has when you don't define a panel background. Otherwise it's pretty useless.