You can save or share your OSX progress photo comparisons directly from the app. Here's how:
On Home, scroll to More and tap Progress Tracker.
Tap Compare.
Select one of the check-in dates.
Choose the two dates you want to compare, then tap Compare.
Tap Share Progress.
Choose which measurement you want to display on the comparison, then tap Share.
From your device's share menu, select Save Image to save the comparison to your phone.
Your progress comparison will now be saved to your device and can also be shared using the available options in your phone's share menu.