There's no UI where you can manage them from a convenient place. From what I found, they're not cleanly stashed in a folder either. They seem to be pulled from the DB with a script when they're requested and "made on the spot".
If you don't really have any MiCollab Client softphones and care only about pictures on 69XX sets, what you could do is change the Avatar setting in MiVB's Online Service URLs form for one of your own web server. I don't know if it absolutely needs to be HTTP but I know that HTTP with Apache works. You'd be able to manage the avatars from a folder on your own website but ONLY for 69XX sets pictures.
The pictures need to be named <extension number>.png and need to have the same file format as those you put in MiCollab itself. If your user have multiple extensions (IP phone, EHDU, softphone, etc.) you need to duplicate the picture for each extension number.
Note that if you leave the ability for the user to modify his own profile picture it might do weird behaviours like displaying the user's picture in MiCollab Client and the server assigned picture on IP sets or some other inconsistency. In short, make sure that pictures are modified only by knowning administrators to avoid problems.
Note also that when you change the URL in MiVB, I noticed that it reset the IP phones background to a solid white but it came back later by itself. So be advised that modifying that URL during business hours on a production system could lead to support calls.