I notice there is no Image Roll Over option when I design as Responsive within Captivate 8. Am I missing something?
Rollovers are not supported for HTML output. Responsive projects are always HTML.
Even for non-responsive projects they will be indicated by the HTML tracker as being not suppoted. They may work on desktop/laptop screens but expecially rescalable projects can also be viewed on tablets/phones and rollover will not work in that case.
Rollovers CAN be inserted in Captivate projects that are non-responsive HTML, but they are only going to be useful if your target audience uses a mouse or touch pad to navigate the content. Users on mobile devices with touch screens are not going to see the rollovers (not even if they wave their hand in front of their touchscreen).
Since responsive design is intended MAINLY for mobile devices, Adobe has taken the decision NOT to include options for adding rollover objects to that kind of content in Captivate.