Dude, you guys are good. I think the examples you posted really do capture those breeds' spirit well.
But, wow, how do you capture a Yorkie's personality? Their "show" look is long, silky hair, expertly manicured with a bow in their hair. I've heard that the best show Yorkies strut in the ring, they are so proud. You'd might check
American Kennel Club - Yorkshire Terrier for the breed standard on coloring, etc.
But most of us with pet quality Yorkies see a different side to them. These guys were bred to chase and kill rats, so Yorkies are ALL DOG. I think all of us at one time or another have described them as "a big dog trapped in a little dog's body."
They are smart, they are curious, and they always seem to be getting into something. They can dig, they can chew, and it's almost as though they knowingly do those things just to mess with you.
I think a pose that would strike a chord with many Yorkie owners would be to have the long-haired show Yorkie, but somehow show the precocious side. Like the hair being intentionally mussed up, maybe even drastically so. Maybe standing on or chewing the bow that it ripped from its own hair. Or another idea, shredding a first-place ribbon that it won in a show.
The head tilt idea fits into that theme because the look is a very piercing, perplexed look, like "Hmm, this behavior bothers you?"
Good luck, I'm dying to see what you guys come up with. I'll go searching for head tilt pictures and post here soon if no one else has.