Add this custom CSS anywhere you can put custom css like your child themes style.css file or a custom css plugin.
#bbpress-forums p.bbp-topic-meta img.avatar,
#bbpress-forums ul.bbp-reply-revision-log img.avatar,
#bbpress-forums ul.bbp-topic-revision-log img.avatar,
#bbpress-forums div.bbp-template-notice img.avatar,
#bbpress-forums .widget_display_topics img.avatar,
#bbpress-forums .widget_display_replies img.avatar {
float: none;
margin-bottom: 0;
vertical-align: middle;
border: 1px solid #ddd;
}
You can use this function (and add it where you can put php code snippets, like your child themes functions.php file or in a plugin like functionality) in this topic to increase those 14px avatars to say 24px.
Resizing Avatars
