[Therion] Maximum surface image size

I created a larger background image for my growing map and now I am getting the following message: therion: warning -- background.th [7] -- error reading bitmap -- unable to determine image size -- images/background.jpg Is there a maximum supported size for JPG? My new background is 15,132x11,340. Or is it possible to input multiple surfaces for a background, so I could break that one large image into smaller tiles? Thanks! Carl