That's only per image. So if you cut up your comic into multiple pages, each up to 1280 px high, then you can just upload them one by one (so page 1, 2 3, etc.) and have them all in one episode without it ruining the flow of your comic. Some people have really big panels and do it panel by panel, others make the cutoff between individual pages (if it's like a traditional graphic novel) but it just depends on how your comic is structured.
The reason you can't upload one giant file is for the sake of website fluidity and not bogging the system down with gigantic file sizes. Could you imagine how slow and clunky the site would get if everyone was allowed to upload 500 MB files that spanned 10,000 pixels, for each image alone? That's why Tapastic also has a size limit - it's so the site can actually, you know, function properyl lol 
(besides the fact that it means consistency across the site, making for a better general appearance regardless of what comic you're reading, which is HELLA important for the user experience).
