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?

Yes, on the iOS version of Twitterrific there's a hidden easter egg that allows you to cycle avatar through avatar shapes to find the one you want. To do this, open Twitterrific's sidebar and tap bottom center icon to open the Theme panel. Next, simply shake your device to cycle through the various avatar shapes until you find the one you want then close the theme panel. Pretty nifty!

NOTE: You cannot yet change avatar shapes on Twitterrific for macOS but this feature is planned for a future update. Stay tuned.