`renderUI` can't find local images when using `flexdashboard`

I have a shiny app that begins with a series of instructions that are conditional on the input that a user selects. I want to include both text and a local image as part of the instructions. renderUI seems to be unable to locate the local image. include_graphics displays the image as expected, but I can not make it conditional on the input that the user selects.

---
title: "MyApp"
runtime: shiny
output: 
  flexdashboard::flex_dashboard:
    orientation: columns
    vertical_layout: fill
---

```{r setup, include=FALSE}
library(flexdashboard)
```

Sidebar {.sidebar}
=====================================

```{r}
selectInput("input_type", 
            label = "Select an input",
            choices = list(" "=1,
                           "Input 1" = 2,
                           "Input 2" = 3))
```


MyApp
=====================================

Row {.tabset}
-----------------------------------------------------------------------

### img src

```{r}
renderUI({
  
  input_type <- input$input_type
  
  if (input_type == 1) {
    HTML('Welcome to my app! Here are some instructions about how to use it. 
A helpful image will appear when you select an input.')} 
  
  else if (input_type == 2) {
    HTML('Here are some instructions.
         <div><img src = "Posit_1.png"></div>')
  } 
  
  else if (input_type == 3) {
    HTML('Here are some other instructions. 
         <div><img src = "RStudio_2.png"></div>')
  }
  
})
```

### include_graphics

```{r}
knitr::include_graphics("Posit_1.png")
knitr::include_graphics("Rstudio_2.png")
```

Start by reproducing the flexdashboard app shown in the issue, comparing renderUI with knitr::include_graphics. Trace how flexdashboard serves local assets through these two paths; done means the conditional Posit_1.png and RStudio_2.png images render for selections 2 and 3 while selection 1 still shows its text.