Have you accounted for the top and bottom margins? Or tried a slightly smaller image to see if you can get them on the same screen.
From the CSS side there is a keep with next or previous property that might help, though I don't know if the reader respect them. And a little search suggests moving the heading inside the div and adding "display: inline-block" to the div. That is supposed to try and keep all the elements in the div together.
You might also ask in the epub section. The experts on doing this are there. To me this isn't really a Kobo question other than getting the sizes right for the different screen sizes.