If you don’t have a picture resizer here is a free one that works great: Villette turned us on to this software and it's easy.
http://bluefive.pair.com/updates.htm
Download Pixresizer
Thumbnails need to be resized to 640X480 pixels
Next, when you are replying scroll down to manage attachments, and find your pictures on your computer. I usually leave mine in my documents after resizing them. Don’t forget to click upload. For your avatar the size needs to be 75X75 pixels.
Hope this helps. There might be an easier way, but I don’t know it….