Hi Luca,
Actually from the user profile interface, there is only the option to upload a new photo, not delete the current one.
But there is an alternative:
1 - If you are trying to delete your own photo as a super user you can go to Host/File Manage;
2 - Check "Recursive" and click "Synchronize Files" - This will ensure that the photo file will show up on the file manager;
3 - Now under "Folders", expand users. You will find a number. Keep expanding until you no longer can.
4 - Select the last folder found on the treeview of folder after having the last folder expanded.
5 - On the right side you should be able to see the picture file that you have uploaded;
6 - Just select it and delete.
That will do it!
Cheers,