Setting custom perspective icons in OmniFocus 2 can only be done using OmniFocus for Mac with the Professional upgrade. Using custom perspectives on the iOS version of OmniFocus also require it have the Pro upgrade.
- In the Menu, go to Perspectives > Show Perspectives
- Click on the Perspective you want to edit, or add a new one. (Note that you cannot change the icons for Inbox, Projects, Contexts, Forecast, Flagged, and Review).
- Drag the icon you want to use from this site to the icon picker.
If you want to use both Mac and iOS icons, the best strategy is to duplicate your perspectives, so you have one you use on the Mac, and one you use on iOS, each with the appropriate icon file.