the information on how to upload an avatar are in the user CP link on the bar at the top of the page. It gives you the size and the format necessary. You either create it in a paint program or you find one on the web.
For signatures, you need a picture already ON the web, on a hosting site that allows remote linking. You have the right tags already (though one per side is all that's necessary), but you can't just put a file name there. it HAS to be a file on the web, with a full http:// web address. For example the sig image in mine is
[ img ] http://mywebpages.comcast.net/cvetters3/fiatlogo2.jpg
[ /img ]
Without the spaces.
I made the picture, then uploaded it to the web using the file storage that came with my ISP (comcast.net in this case). There are many you can use, but a lot of them don't allow remote linking.