Totally understand what you're going through trying to figure out how to post horizontal comics on Tapastic's vertical design. It's actually not so jarring as you may think to read a horizontal comic in an infinite canvas setup. With the infinite canvas, it actually makes the transition/loading of a new page so much more seamless and easier for readers. My comic is actually created in a horizontal format and i post both that version of my comic and a vertically formatted version on another series so readers can subscribe and follow whichever format they choose to read in. (currently the vertical format version is the more popular one) A lot of readers here read comics on the mobile app which the vertically formatted comics (mobile optimized) tend to look better on.
Here's what I mean:
Oops in Standard Format - http://tapastic.com/series/OopsComicAdventure
Oops in Vertical Format - http://tapastic.com/series/Oops-Comic-Adventure-3
Also check out this forum thread that @elixiadragmire started on Comic page layouts (*sneaks a cookie, mmmm coookies)
She's put a lot of thought into the thread post and has even created simple helpful graphics to help explain. It's really been very help for me.
No worries. You can post splash pages, it's just the doesn't have quite the same impact.
Here's an example of what I mean. In my comic, Oops Comic Adventure, in issue 3 at page 23/24 is a splash page:
And here's how I took the same page and vertically formatted it:
It takes some creative thinking to take a horizontal comic and reformat it for vertical, however, it can sometimes lead to a different, more surprising reading experience, since the readers can't see the whole page all at once and they have to scroll to get the reveal.
I also recommend checking out Tapastic's Creator Guide series. They have an episode all about vertically formatting, and in the comments you can read all our growing pains about it (even mine)