I haven' t used the visitor message much myself so I am not too familiar with it either. I just got my first ipadbthree weeks ago and it is the iPad 3. I have never used one before and never had an iPhone either so there is a lot I don't know, lol.
There is no minimum number of posts to have an avatar. It sounds like your file is too big. It can only be 80 x80 pixels and 19.5mb. Can you reduce the size of the file?