Getting the web image saved will vary with your OS. What I did on Windows was get the webpage on the screen, hit Shft-Ctl-PrtScn. Then I opened up MSPaint and hit Ctl-V to paste it. Then I cropped it and saved it as LaTeXwiki.jpg.
Once I had the image, that's where LaTeX comes in. I used
\stackinset to overlay my comments upon the image. In this case, I used three nested
\stackinsets laid upon the imported graphic. I used the left-top of the image as the origin (the
t arguments of
\stackinset); however, each inset could, if desired, specify its own origin (l,c,r for horizontal, t,c,b for vertical).