Though Instagram doesn’t have a default profile picture, you can delete yours and have a blank avatar by selecting Edit Profile > Change Profile Photo > Remove Photo.
To set up the default Instagram profile picture (PFP), go to your profile, tap "Edit Profile," then tap "Change Profile Photo." Select "Remove Current Photo" to revert to the default blank avatar provided by Instagram.
Instagram lacks a default profile picture which one can manually set. When you would like to have the default appearance, just delete the photo in your profile:
Click on your profile and then go to Change Profile Photo.
Select Remove Photo.
Your profile will be moved back to the default avatar of Instagram (a gray figure).