### Bug description

I tried to render a reveal.js presentation into pptx instead and found an error. I created a repo to demo the error here: <https://github.com/anielsen001/quarot-pptx-bug-000>. 

The problem is that I have a background-image on a slide like this:
```
## Quarto {background-image="/img/quarto.png" background-size="contain" background-opacity=0.1}

Quarto enables you to weave together content and executable code into a finished document. To learn more about Quarto see <https://quarto.org>.
```
The reveal.js render works fine, but when rendering to pptx, it cannot find the image, and i get this error message:
```06:56 $ quarto render
pandoc 
  to: revealjs
  output-file: pptx-bug-1.html
  standalone: true
  wrap: none
  default-image-extension: png
  html-math-method:
    method: mathjax
    url: >-
      https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js?config=TeX-AMS_HTML-full
  slide-level: 2
  
metadata
  link-citations: true
  width: 1050
  height: 700
  margin: 0.1
  center: false
  navigationMode: linear
  controlsLayout: edges
  controlsTutorial: false
  hash: true
  history: true
  hashOneBasedIndex: false
  fragmentInURL: false
  transition: none
  backgroundTransition: none
  pdfSeparateFragments: false
  lang: en
  auto-stretch: true
  title: pptx-bug-1
  
pandoc 
  to: pptx
  output-file: pptx-bug-1.pptx
  default-image-extension: png
  
metadata
  title: pptx-bug-1
  
pandoc: /img/quarto.png: withBinaryFile: does not exist (No such file or directory)
```

I believe it has to do with how I specify the path to the background image, but it works for reveal.js. I'm not sure how to specify it for pptx rendering. I think that the syntax for one should work for both. I'd like to be able to simply add a new type of output and have my existing content just work. 

I'm using quarto version 1.3.340 on the command line, on Ubuntu 20.04. here's the output of `quarto check`:
```
07:00 $ quarto check

[✓] Checking versions of quarto binary dependencies...
      Pandoc version 3.1.1: OK
      Dart Sass version 1.55.0: OK
[✓] Checking versions of quarto dependencies......OK
[✓] Checking Quarto installation......OK
      Version: 1.3.340
      Path: /opt/quarto/bin

[✓] Checking basic markdown render....OK

[✓] Checking Python 3 installation....OK
      Version: 3.8.3
      Path: /home/apn/sw/bin/python3
      Jupyter: 4.10.0
      Kernels: julia-1.8, python3

[✓] Checking Jupyter engine render....OK

[✓] Checking R installation...........OK
      Version: 4.2.3
      Path: /usr/lib/R
      LibPaths:
        - /home/apn/R/x86_64-pc-linux-gnu-library/4.2
        - /usr/local/lib/R/site-library
        - /usr/lib/R/site-library
        - /usr/lib/R/library
      knitr: 1.42
      rmarkdown: 2.20

[✓] Checking Knitr engine render......OK
```




### Checklist

- [X] Please include a minimal, fully reproducible example in a single .qmd file? Please provide the whole file rather than the snippet you believe is causing the issue.
- [X] Please [format your issue](https://quarto.org/bug-reports.html#formatting-make-githubs-markdown-work-for-us) so it is easier for us to read the bug report.
- [ ] Please document the RStudio IDE version you're running (if applicable), by providing the value displayed in the "About RStudio" main menu dialog?
- [X] Please document the operating system you're running. If on Linux, please provide the specific distribution.
- [X] Please provide the output of `quarto check` so we know which version of quarto and its dependencies you're running.
cscheid
2023-05-11T11:02:13Z
anielsen001
bug powerpoint
v1.4
open
background image in pptx render not found
https://github.com/quarto-dev/quarto-cli/issues/5498