Sorry to dredge this thread up... but I am having the same issue (not being able to add JS) and the advice given does not seem to be working in my case...
The problem looks a little like this...
Correct, working code
<ul class="hoverbox">
<li><img src="assets/specimages/_resampled/ResizedImage10067-3.jpg" alt="description" onmouseover=show_200("assets/specimages/3.jpg","image_3",418,280) onmouseout=hide_div() onerror=gag(this,100) ></li>
</ul>
The Code after SS/TinyMCE is finished with it.
<ul class="hoverbox">
<li><img onmouseover="show_200("assets/specimages/3.jpg","image_3",418,280)" onmouseout="hide_div()" src="assets/specimages/_resampled/ResizedImage10067-3.jpg" alt="description" ></li>
</ul>
What you see there, is still the result after commenting out the lines recommended by Banal.