When you go to a web page your computer (depending on your settings) will save the images. It allows the page to load faster and reduces the amount of bandwidth needed for you and the host. When you went back to the site after changing your avatar your computer was showing you your saved image. If you'd have taken a look at the address of the one that was in the avatar screen after you posted it there you'd notice that its url was subtly different from the one forum seems to dedicate to us for our avatar which is always the same. Hence the two different images. I gave you your link. You went to it (with it still showing you the image your computer saved), deleted it from your hard drive and made your browser reload the image by pressing ctrl_f5. Also known as clearing the cache. Because you went to that link it only reloaded that one image. If you'd have done it on the forum (in this post for example) it would have reloaded all of the images. Not fun if you have a slow connection.
You can do it by just pressing f5 sometimes, but it doesn't always work. I know not why. If you're using Opera pressing f5 should work (you can also use the "show images" button, press it three times I think). If you press ctrl_f5 it will reload ALL of the windows that you have open. Not fun if you've got text in a box or something because you'll lose it.