Adding button cuts off background on slide

I'm learning Captivate and ran into a problem I can't solve. I wanted to control slide advancement with a click, so I added a button. When I HTML5 preview the project, it shows only part of the page before I hit return (which I coded in as an alternative to clicking). Then it shows the whole slide, but advanced on time to the next slide. 

Before click:

After click:

I removed the button and it worked fine (except it advances on time, rather than a click). When I put in a new button, the problem showed up again.

Is there something I need to do when putting in image buttons? I have text buttons elsewhere and they work fine (although I haven't quite figured out the timing elements—more reading and practice for me!)