You do not need a plugin.
Here is what you do:
Using FTP or online Administration Control Panel that your hosting company provided to you:
XxSky DreamerxX Wrote:Go to your root folder: public_html > forums > images > default_avatar.gif
Now, in where the public_html is the directory where your whole website is located.
Forums is where your mybb forum is located. It could be forum, forums, or anything that you named it.
Then inside it, click on "images" folder.
Replace the default avatar with the avatar that you want and make sure its
gif.