On Panka you have two photos. The one your team-mates see is your player card photo: go to “Profile” → “PLAYER”, tap your player card and choose an image from the photos on your phone. You change your account photo from “Profile” → “ACCOUNT”, with the camera icon. In both cases it is saved straight away.
If “Gallery permission denied” appears when you tap the photo, you have denied Panka access to your photos. Your phone usually doesn’t ask again: turn access back on from the phone’s settings (on iPhone, Settings → Apps → Panka → Photos) and try again.