Do you get the same error if you use the Tinymce editor, or the No Editor option?

All the rollover feature does is add an onmousover and onmouseout attribute to the <img> tag, eg:

<img onmouseover="this.src='/images/responsive/winter-hd.jpg';" onmouseout="this.src='/images/responsive/winter.jpg';" src="https://www.joomlacontenteditor.net/images/responsive/winter.jpg" alt="winter" width="400" />

The reason I was asking about this with the other editor options, is that I think the error is caused by an extension you have installed or something on your server, which is rejecting the save request because of the onmousover and onmouseout events - they may be blacklisted because in some case they can be used for malicious purposes - and testing this in another editor would confirm this.