In the Nyan Cat group? If so, when your on the main part of the group(like you see the discussions,the members) there should be a button that says "Add Pictures",then chose the pictures. Well, in the discussions, in the toolbar(when you're typing something), you see the images that have the mini earth,email thing,the small image,and what looks like a strip from a movie. Click the second to last button and paste the URL in the box. In order to get the URL, you have to use something like Tinypic,Photobucket,ImageShack,etc.
Hope that helped.