We have some custom code to show the OP avatar on the mobile view. No complaints. Feel free to check it out at www.helloforos.com
On the site homepage, it shows the category name below the post, but if you are in a category it displays the author’s name.
In a small community, I understand the idea of showing the latest reply avatar. In a large community with lots of activity, I find it’s helpful that the community see who originally wrote the topic as it is part of the decision of whether users want to enter or not. I believe it also provides better recognition to authors of quality content