I don't know if it's a stumbling block or not--depends which side of the psychological fence you sit on, I guess.
For myself, I don't try to picture myself "thin" or even "thinner". Basically, my idea of what I "will look like" is just...like me.
What I try to do, instead, in terms of visualization is imagine myself dealing with challenges that I know I will face in ways that make me feel good about myself. I'll give you an example.
This last sunday, my best friend and her parents held a Christmas party at her parents' new house. I had been helping them prepare (slicing veggies, pulling the necessary dishes, etc) so I knew exactly what was on the menu. I knew, for instance, that there were going to be TWO BASKETS and THREE PLATES of brownies and sugar cookies. Brownies are my weakness, I avoid them at all costs because once I start, there's no stopping me!
So anyway, I knew there were going to be brownies there. I repeatedly told myself that I wouldn't even start, I would spend my time chatting to others and helping where I was needed. I imagined myself having NO BROWNIES, and I imagined how good I would feel if I could go home at the end of the party knowing that I had held true to that goal. I spent three days preparing myself for this.
And I did it! The whole party I had one piece of broccoli from the veggie tray!
I was so proud, and my reward was losing a pound this week and making my Christmas goal.
I do believe that I couldn't have done it if I hadn't envisioned myself being successful. I guess I just have a more immediate, rather than a more global, perspective.