Customizing Avatar Shapes in the Timeline (iOS and macOS)

Is there a way to change avatar shapes displayed in the timeline from square to something else?

On iOS, open Twitterrific's sidebar and tap the bottom center icon to open the Theme panel and select the shape you want. (Easter egg: If you shake your device while the theme panel is open, you might find some other shapes, too!)

On macOS, you will find the avatar shape option by opening Twitterrific's Preferences and selecting the Appearance panel.