Attached Link: http://www.freeverse.com/comiclife/

Are you using a Mac? If so I highly recommend "Comic Life" -- very inexpensive, works well with iPhoto. There are probably similar apps on Windows (but I don't know them)

There's nothing to stop you doing it all in Photoshop, though. You create your document, of the appropriate size (eg for a 2 page spread, its 11 inches high by 17 inches wide). You then use Photoshop's "Place" command to bring external files into this new document.