Welcome to XF2 Addons. All the XF2 addons located in the Resources area are available for download once you upgrade your account to Premium membership. In addition, Premium members will get technical support. Premium membership costs only $25.00 USD a year. Once the year has expired you can continue to use the installed addons on your forum as long as you like.
You can do this with notices. Just create a notice - either popup (floating) or at top of forum (block or scrolling) with user criteria of has no avatar. Easy peasy. I prefer the look of a block or scrolling block - less distracting that a popup (floating) and allow it to be dismissed so they only see it once.