How can I create a Rollover Image in the Cap 2019?

Just a warning because I watched that video:

• Author of the video forgot to disable the Click sound on the interactive shape button. Messages and actions were disabled. 
• This was demonstrated in a Fluid Boxes project, but the shape button was NOT inserted in a fluid box, which means that when the browser resolution and/or the device is different from the desktop resolution shown, that the rollover image can appear any where. Reason: you cannot have this setup in a normal fluid box, it needs to be a static fluid box which was (of course) not mentioned.

I would indeed have answered that a multistate object, interactive so that you have an automatic InBuilt state for Rollover is the way to approach this, but you will have problems when trying to do it in a Fluid Boxes responsive project. It works perfectly for non-responsive projects if  you publish then with Scalibility option for HTML output.

CP2019 covers a lot of versions, when posting a question it is wise to mention the full version number as you can find under Help, About Captivate.

Have a look at some tips about states in interactive objects:

https://blog.lilybiri.com/button-states-basics-and-tips