Look at Daystyle Designs - Leah Day's website.
http://www.daystyledesigns.com/pebbling.htm I love pebbling freehand on my Janome 6600 - the backtracking can even be used to add definition and I often back track on purpose to this end. Also, use a fine thread if you don't want the backtracking to dominate, I have used Bottom Line and also YLI 60 wt. However a 50 wt gives more definition if this is the effect you want. This on my Ammonite was done with Masterpiece 50 wt. Another key element when pebbling, to make it look naturalistic is to ensure you round out every pebble and also make sure that you vary the sizes. It's like a jigsaw, filling the gaps with no spaces. Like all other FMQ - PRACTICE! :-D