Updating.
Today I found a solution, i´m not sure if this is the best one, but it works for me.
Trying to understand what is happening with the image adjustment of the banner, I did the following steps:
1 – I made a print screen of a 100% visualization of the home page on my screen (Focusing in the banner area).
2 – I pasted the print screen in the photoshop and cropped only the banner area and then I go to the image informations to see what is the real size of this banner when its appear on the screen.
Curiously the size is 1006x404px, kind different from the 1600x400px that the option panel shows in the instructions.
3- So, I have resized my image to 1006x404px and uploaded into to the panel, and works perfectly.
Take a look, its not distorted.
