If you use Google Chrome as your browser here's an easy way to add highlighting to a page.

  1. Go to the page you want to screenshot with some highlighting
  2. Select the text
  3. Right click on the page and click on "Inspect element"
  4. On the right hand side of the elements panel you'll see "Styles"
  5. Click the "+" button
  6. Add the following code:::selection {
    background: #FFFF00;
  7. Screenshot away!