When you are adding an expense in the offline app, a new button will appear at the bottom:
Clicking the "Add Receipt..." button will pop up this dialog (on Android 3.0 and higher):
You can then take a picture of the receipt and save the expenses. If you are offline, the app will sync with Torch once you are back online.
This is implemented with the new media capture api, which is a w3 standard. So, at the moment it only works with Android, but it should be coming to other platforms as well.