if you have a 100 Mbit LAN then 7 MB/s is about the maximum you can achieve, but even on a Gigabit network the speed will not be above 10 MB/s due to the limited RAM en cpu that the spd8020 has.
Also the size of the files you upload is of infleunce. Many small files take more time then one big file.
of course you can copy the files by connecting the disk directly to a PC, its up to you to decide. You will find on sdb6 folders with names equal to the shared folders you defined. However I doubt if it will be much faster for files of size of a few MB (pictures). I would just continue to upload them via the LAN