Is it possible to use a (locally-sourced) full width image in place of the title banner?

[petrelhead]

Thank you for making posterdown ! This is more a general question than an issue - hope that is ok!

I am trying to use posterdown to recreate a leaflet style A4 size document with a full-width header and 2 columns of content (it's a bit of a hack, but appears to be my best chance of doing this in R...?). I need to reproduce this document annually with updated results. I have previously had to generate the outputs in R but collate them all in Word. The doc requires combinations of text and figures in 2 columns and a header (banner) title with logo at the top that spans across the whole width of the page. I have this full-width header image with logo embedded saved as a .png file and I would like to be able to use it in place of the current topsize and title parts of the poster template. I have removed these existing parts of the poster, so that my content starts right at the top, but can't work out how to include a full-width banner image at the top (that I source from .png) - followed by the column formatting below. Is this possible?

I am using the posterdown_betterport template, but I guess this query could relate to all templates really. I am using windows 10, R version 4.0.2, and R Studio version 1.3.959. I am using the github version of posterdown.

What I have tried so far is removing the title and standard header and then adding my own image as the first thing on the poster - but this doesn't seem to allow the image to span the full page width (it only displays in the left-hand column and is then cut off...

main_topsize: 0 # remove the standard header and footer
main_bottomsize: 0
# title: '**A Better Reproducible Poster Title**' # Remove title - all content should push up to avoid having white space where title would have been.
poster_height: "29.7cm" # Change to A4 size
poster_width: "21cm"
main_textsize: "12pt" # Adjust text size so it is suitable for A4 output
body_textsize: "12pt"
title_textsize: "30pt"
caption_textsize: "12pt"
column_numbers: 2
output: 
  posterdown::posterdown_betterport:
    self_contained: false
    pandoc_args: --mathjax
    number_sections: false
bibliography: packages.bib
link-citations: true
---

```{r, include=FALSE}
knitr::opts_chunk$set(echo = FALSE,
                      warning = FALSE,
                      tidy = FALSE,
                      message = FALSE,
                      fig.align = 'center',
                      out.width = "100%")
options(knitr.table.format = "html") ``` 

![alt text](Header_and_title_bar_square.png) # Try including my full-width .png file that I want as the header image 

Thanks in advance.