Hi Bastien. The image intro plugin in X3 is
only for setting the top intro image. Although it works technically similar to "parallax" plugin, it's a different implementation ... The image intro is simply an image (not a background), while "parallax" is often used to set "background images" into the page content, where the background image scrolls upwards at a fraction of the scroll speed.
I see you have already implemented the basic idea. Good job :clap: I guess you wanted the background to be full browser width? And perhaps a fractional scroll on the background? Unfortunately, this would be a "plugin" I would have to create specifically, and not something I could explain in a forum post. Additionally, to get the background at full browser width, essentially you must set your context module to WIDE width, and then instead use a grid within to limit text width. As it is now, you have placed your parallax inside context section which is already limited by a width, and it can't expand to 100% as long as it's inside the context (unless context is set to 100%).
You could try to add some negative margin to your parallax css to make the image slightly wider:
I'll consider this as a plugin in the future, but I can't provide much more help for now