Since images in LDAP are binary data / BLOBs, and the Alfresco element for user's avator is managed via a child-association, which isn't handled in the same way that any other, trivial properties are, this is plain not possible using any configuration with default Alfresco. You will need to use addons for this. In one of my addons I once added support for handling photos in LDAP sync, but to be fair, I haven't updated that addon in a long time to be compatible with newer Alfresco versions (I no longer work with the customer for which I originally created that addon, and there hasn't been any other interest from the community). I know that others have also implemented similar functionality, e.g. with a quick Google search I found a config snippet in another module that looks to support avatar synch.