I have a Longchamp backpack that I use when I travel. The shoulder straps pull up to tighten the opening, so no one can get into it without me feeling it. (unless they cut it with a knife -- that would be so rare anywhere)
Mostly if I used it to take photos, I held it in my hand. Never put it on a table when eating, and only kept it in back pocket of my jeans (like I do all day at home) if I was not around other people. I tend to use a regular camera, but did use my iPhone when I wanted to Instragram a photo, or Facebook one instantly.
All that being said, you really need only worry if you are in a big group of people, crowded together, like on a bus or metro.
I've experienced one attempt at being pickpocketed, and it was of course on the Metro. The man didn't get us because I had all our valuable in my over the shoulder bag, and my hand on the zipper...
I'd recommend a shoulder bag of a small size that has a zipper, or some sort of flap, and if in a crowded situtation, just make sure you keep one hand on the opening. I promise you, you will NOT feel the pick pockets fingers working your bag open, no matter what type of bag it is.
To be very secure, the best would be a PacSafe bag....they have locks on all zippers.