<style>
img{
    font-style:italic;
    color:#c00;
}

/**
 * In Firefox we can insert the string ‘( .... )’ into the alt text of an image that hasn’t loaded.
 */
img:after   { content:" (Image - Right click to reload if not loaded)"; }
img::after  { content:" (Image - Right click to reload if not loaded)"; } 
</style>

In Firefox and Chrome (and possibly more) we can insert the string ‘( .... )’ into the alt text of an image that hasn’t loaded.

img {
  font-style: italic;
  color: #c00;
}

img:after {
  content: " (Image - Right click to reload if not loaded)";
}

img::after {
  content: " (Image - Right click to reload if not loaded)";
}

<img alt="Alt text - " />