It seems like it might be simpler both for the dev and the user to just send an SMS with a neatly-formatted text version of the bill rather than a link to a screenshot of the bill. If it's just a couple amounts, tax, tip, and total, that seems like it would easily fit in 140 characters. The app design looks pretty nice though.

Edit: I guess maybe the advantage of this design is that you can easily send the bill for everyone at the table to everyone (probably wouldn't fit into 140 chars), which eliminates the need for the user to individually choose which contact corresponds to which column of the bill.