This is not exactly what you want but then it helps me to solve a similar issue, in this (
https://apsic.scicore.unibas.ch/ ) project in the sidebar as you can see there are some little png pics that are approximately 150*150 pixels.
On the server side I used an HTML-like code as follows:
output$imageBody <- renderImage({
if (input$cancer == "Breast:Carcinoma") {
return(list(
src = "body/breast.png",
contentType = "image/png",width = "100%", height = "100%",
alt = "Breast"
))}
)}
It is free of icon size limitations and it is easy to convert pictures of other formats into png same size.
I hope it helps
BR
Ehsan