Hello FlLady20,
You did a good job isolating the issue to the specific app on your iPhone. If you know that rotating a picture is a feature the app is capable, here are some steps to take:
- Close and reopen the app
- Restart your device
- Check for updates
- Delete the app, then redownload it
Specific instructions for each of those steps is available here: If an app on your iPhone or iPad stops responding, closes unexpectedly, or won’t open - Apple Support
If you are still not able to rotate the photos, the next step would be to reach out to the developer using the steps found here: How to contact an app developer - Apple Support
Have a good day.