@helge,
To allow users to add or edit their profile picture (avatar) in wpForo, make sure they have the "Edit Account Information" button available on their profile page. If users don't see this button, check the setting in your WordPress Dashboard under wpForo > Settings > Spam Protection. There is an option called "Min number of posts to be able to edit profile information"; adjust this setting so users meet the requirement to edit their profile, including their avatar. This will enable users to upload or change their profile picture (avatar).

@helge,
The recommended size for profile avatars in wpForo is typically 42x42 pixels. You can adjust the avatar size by adding custom CSS in Dashboard > wpForo > Settings > Colors & Styles > Custom CSS, for example:

#wpforo #wpforo-wrap .wpfl-3 .wpforo-topic .wpforo-topic-avatar img, 
#wpforo #wpforo-wrap .wpfl-3 .wpforo-topic .wpforo-topic-avatar img.avatar {
  width: 42px !important;
  height: 42px !important;
}