Hello Henri,
I am really sorry for the delay of this reply as it is a weekend for us here. I believe it is EasyBlog category that you are trying to change and not EasySocial page because in your screen shot, that seems like a category in EasyBlog.
To change the avatar for the category in EasyBlog, go to the Categories section at the back end of EasyBlog. Then, click on the category that you are trying to edit and upload the avatar as you can see here,
http://take.ms/wpQM1
Then, save your category and you are good to go