Changing your avatar really isn't that hard. One thing you must do first, however, is have the picture you want to use already located on your computer. It should be a picture that you own, or have found on the net with a copyright of "Creative commons". It has to be already on your computer for the next step, so if you go find one on the net, be sure to copy it to a folder you can easily browse to, such as your "My Pictures" directory or desktop.
Once you have done that, follow these steps:
At the top of any Newsvine page is a menu bar, and one of the selections is "Customize Column".
Click that, scroll down until you see "About [your username]". Click "edit".
You will now see your old avatar (picture). Make sure the popup blocker in your browser is off. Click "upload" or "change/remove" (whichever of the two buttons is showing), and a small pop-up window will be displayed.
Click the "browse" button, then choose the graphic you want to upload. After you click the graphic you want, click "Send File".
On the next screen, there is a slider you can use to resize the image (if needed). Then click "Save". Close the little window, click "Save your profile", and you're done!