Im no coder, but I would look at how others do it. phpbb has this avatar function built in, and it seems to work pretty well. http://www.phpbb.com

To manipulate the size of the picture, you will need to have the NetBPM library installed and running on the web server you are using.

When you start talking about letting users add files, then you will most likely have to disable some security features like safe mode.

Sorry about the non specific answer, but hopefully this will make you ask more questions and eventually lead to the correct answer.