## Get the size of the image file using Lua.
## Cf. http://wiki.nginx.org/HttpLuaModule#set_by_lua.
location ~* \.(?:gif|jpe?g|png)$ {
    set_by_lua $img_file_size '
      function fsize (file)
        local current = file:seek()      -- get current position
        local size = file:seek("end")    -- get file size
        file:seek("set", current)        -- restore position
        return size
      end
      fsize(ngx.var.request_filename)';
}