That OP example renders fine for Topic List Previews, because I use broader criteria in the plugin:
def extract_images_for_post
# all images with a src attribute
@doc.css("img[src]") -
# minus emojis
@doc.css("img.emoji") -
# minus images inside quotes
@doc.css(".quote img") -
# minus onebox site icons
@doc.css("img.site-icon") -
# minus onebox avatars
@doc.css("img.onebox-avatar") #Broader criteria than Discourse Core
end