By transferred you mean took them completely off your computer or just copied over?
If you just copied them over (and still have the original files on the computer) just highlight each one, hit triangle and hit "delete" to remove them.
If, however, you actually moved them completely off the computer just plug in your memory stick/ thumb drive, highlight the pic, hit triangle and select "copy". You may be able to make this process quicker by hitting the square button to group everything together in the one folder (the PS3 will sort by when they were put on the system) and just copy that folder. Then you will be free to delete them off as I described above.