title: How to do in R: load an image file, print text on image, save modified image
body: <p>... Ideally, with a PNG image file, if not JPEG. I can load a flie with</p>
  
  <pre><code>library(png)
  img = readPNG("chart.png")
  </code></pre>
  
  <p>but am stuck there. The goal is to type "Hello world" on the image and save it.</p>
  
  <p>Thank you.</p>