Edit
Aside from standard features like mark, delete, copy, paste and cut that are applied to the selected frames of the timeline, you'll find the following specific functions in the menu:
Reverse order
This reverses the order of the selected frames in the timeline.
Edit in <external application>
Opens the selected frames directly in the application you have set in the preferences (e.g. Adobe Photoshop). There you can edit frames individually or batch process them. After saving from the external application the files will be re-imported into iStopMotion.