Yes of course I did try to ad a "with=800px" or similar to the Section of this container. But I not succed.

<section class="mt16 mb16 oe_img_bg" style="height: 152px; overflow: hidden; background-image: url(https://mysite/website/image/ir.attachment/172_ad786a5/datas);">    

 <section class="parallax oe_img_bg" data-scroll-background-offset="0" data-scroll-background-ratio="0.1" style="height: 312px; overflow: hidden; background-image: url(https://mysite/website/static/src/img/library/ipad.png); background-attachment: scroll; background-position: 0px 0px;">
   

The hight here is not the image but the height of the container. First example is just a text container with a background image, the second a vrtical parallax.

Can't you just force the change in the HTML editor ?