how to download Rando photos from iPhone to Mac
Any on screen image can be captured in a screenshot (press and hold home then press lock) which will be saved to your local camera roll.  If Rondo does not support exporting photos explicitly, you can just create a local copy using this method, the only downside being that the resolution will be capped to your devices screen size, not the original photo dimensions.

After actually checking the app out, I noticed that if you double tap an image it gives you the option to save to the camera roll anyway, so you then transfer via the method of your choice: Photostream/iTunes Sync/Email/iMessage etc