When you do isolation exercises for the delts you need to keep tension on them at all times. The best coach I ever worked with had all of us contract at the top of a movement for three seconds. This especially works well for Bent over cable raises and the exercise Prof X mentioned. Try to lower the weight a bit and do sets of 8-12 reps (start out with 2 sets each of two exercises and work up to 4 sets of 12 eventually).
Do these after some heavy rowing (palms down and bring barbell to the middle chest area) and after a month or so incorporate some of Waterbury's tips and start working them out 2-3-4-5 times a week for a while with different set and reps schemes.
I myself am doing this at them moment for shoulders and they are blowing up!
Good luck. Your development looks good. Keep it up and post another picture in a few months to let us know your progress, OK?