Outlines on low-res picture

Hi,

i want to have outline to use it on a plotter to make stencil. I use to make stencil every week, no problem...

...but i lose so much time to make the outline path on illustrator because i do not find the right trick to do it.

I use very low pixelated picture (182pxX 196px/ 12,7ppi / 14,331 inches X 15,433) because i draw in photoshop like that and i can't change that. 

My technic for now in illustrator :

1/ i make a mosaïc

2/ i select the purple part and delete it

3/ ...and delete all the paths remaind in the form i want to keep (picture below) 

i want to create the outline of all forms (the green one) without all inside squares.

May i ?

Thanks! 

greg