If that is the case I'd try a different approach. Use a utility programme to download the webpage or create a local copy. Then do your edits with highlighting and final step is to create your print-screen or jpg copy.
The hardest part about what you are trying to do is that it will be time intensive to select the text then apply a highlight. If you can do the highlight to live text you will save yourself huge amounts of time.
I know that Adobe Acrobat Professional will allow you to create a PDF of a web page and you would then be able to manipulate that PDF in other programmes. I'm sure others can suggest other tools that may do the same thing. Certainly if you have the ability to work off the live text version I would strongly recommend that as the starting point.