BuddyPress.org

It’s simply a case of understanding the weight of selectors, now you have an id on the ul you should be able to weight the selector set in your favour i,e:

#avatar-block-inline li.item-avatar-online a img {}

if necessary add classes to the a or img elements

changing the class names as I have may well have removed some of the styling that was being applied so you may not have the issue you did, but also may have to style anew as it were.