That would need few code changes which cannot be explained here. What it would need is to get the last answerer's id and get the avatar. However that is not efficient way. Instead, we should create a column in jos_answers_questions namely last_answerer and insert the answerer id everytime a new answer is added. So it can be easily retrieved without querying additional DB queries. Adding last answerer part is being done currently so you need not worry about it, only change you need to do is change user avatar to answerer avatar.
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