Pages (desktop), now with Export to ePub, of course <
http://support.apple.com/kb/HT4168>; Page Setup set to Tabloid Extra; all margins set to zero; care with the Edit Mask function.
800 x 1236 PNG files go full-frame, no prob. No jaggies; no cutoffs; no blank pages.
Trying to do a Table Of Contents, bollixes all that though. (Perhaps a page of just text, ahead of the chapter.)
Be sure to check the box for "Use first page as cover image," in the export dialog.