It's the image cache. To save time Browsers put images in a cache and load them only once. If the pathname doesn't change then the image won't change, even when you do change it. Your old image was at http://www.evcforum.net/Images/Avatars/5742.jpg, and your new one simply overwrote it.
So just erase your image cache:
Go to Google Chrome's "Settings" page.
Click on "Clear browsing data..."
Make sure only the checkbox for "Cached images and files" is checked.
Click on "Clear browsing data"
If that doesn't work then near the top of the "Clear browsing data" page you'll see it says, "Obliterate the following items for ", select a time period that goes back to before you changed your avatar, then try again.
The cache is updated periodically anyway, I don't know how often. Love the new avatar.