Randomized loading page image in Shiny App - css

Question

I have a Shiny app that has several reactive tables that take time to load and have successfully modified the code I found on this website(https://www.listendata.com/2019/07/add-loader-for-shiny-r.html) to have a loading gif display when each tab is selected as the output tables generate (but not when conditional panels are generating).
I would like to randomize the image. I suspect this can be achieved with the shinyJS package but I have very little Java experience and I just can't seem to make sense of it.
library(shiny)
library(shinydashboard)
# User Interface
ui <-
dashboardPage(skin = "black",
dashboardHeader(title = "Loading Screen"),
dashboardSidebar(),
dashboardBody(
# Javasript Code
singleton(tags$head(HTML("
<script type='text/javascript'>
/* When recalculating starts, show loading screen */
$(document).on('shiny:recalculating', function(event) {
if(event.target.id === 'plot'){
$('div#divLoading').addClass('show');
}
});
/* When new value or error comes in, hide loading screen */
$(document).on('shiny:value shiny:error', function(event) {
$('div#divLoading').removeClass('show');
});
</script>"))),
# CSS Code
singleton(tags$head(HTML(paste0("
<style type='text/css'>
#divLoading
{
display : none;
}
#divLoading.show
{
display : block;
position : fixed;
z-index: 100;
background-image : url('http://northerntechmap.com/assets/img/loading-dog.gif');
background-size: auto 40%;
background-repeat : no-repeat;
background-position : center;
left : 0;
bottom : 0;
right : 0;
top : 0;
}
#loadinggif.show
{
left : 50%;
top : 50%;
position : absolute;
z-index : 101;
-webkit-transform: translateY(-50%);
transform: translateY(-50%);
width: 100%;
margin-left : -16px;
margin-top : -16px;
}
div.content {
width : 1000px;
height : 1000px;
}
</style>")))),
# HTML Code
box(tags$body(HTML("<div id='divLoading'> </div>")),
plotOutput('plot', width = "800px", height = "450px"),
actionButton('goPlot', 'Generate Plot', icon("paper-plane"),
style="color: #fff; background-color: #337ab7; border-color: #2e6da4"),
width = 12, height = 500)
))
server <- function(input, output) {
output$plot <- renderPlot({
input$goPlot
Sys.sleep(3)
plot(iris$Sepal.Length, iris$Petal.Length)
})
}
shinyApp(ui, server)