I was surprised when I first signed up to join the MOODLE community that users could upload personal icons on an educational site.
I was going to suggest that a choice of icons be provided to students but I forgot all about it until this thread. The icons could be listed under different categories for example, the Simpsons, the Lord of the Rings, Aliens etc. Instructors and Administrators could have the choice of a web site icon or their own.
I also feel education sites should not be open to the public. This may not stop all mischief but much of it. Remember! Not all mischief is done by students at your school it may be done by students from another education facility. A lot of mischief is also done by adults.
I am not that acquainted with postNUKE. I am more acquainted with phpNUKE. In the "Your Account" module of phpNUKE a site member can choose an icon from an icon page. Usually all one has to do is click on the icon image and it is place on the "Your Account" page. Many phpNUKE sites allow members to upload images or link to them from other sites as well but for educational sites with young students I would not allow it. Only admins and teachers would have this type of privilege.
You can give your self less (not totally avert) of a headache by allowing students the freedom of choosing teacher pre-selected icons while maintaining some kind of control.
It's a bit of a "pain in the t....." and a lesson lost in responsibility for the students but uploading a list of students and teachers to the site while eliminating online signups can decrease your worries. There are some "free" online tools to also help you generate passwords for each student to make it a bit easier to create accounts and therefore, pre-enroll students.
When I used an online grading and attendance program for students, I used the "Merge" function in a wordprocessor to create student lists. I created a table with the names of students and their class, constructed a letter to their parents notifying them of the site (and the fact that they could check the students' daily grades and attendence) and providing them with the UserID and Password to the site for their child. I also provided students with the information the same way. Takes a bit of time but again makes the worries less. The Merge functions personalizes the pages for the parents and the students by placing the parents names and the students names on individual letters.
Hope this encourages more discussion on this important matter.