Tilly is absolutely adorable, no matter what she is. If you want to be sure, though, you can send out a dna sample.
Dog DNA Test Doggie DNA Testing and Mixed Breed Canine DNA Testing For Dogs - Dog-DNA.com
Another YT member had a dog that 100% of YT members that guessed said absolutely could not be a pure yorkie from the pics, and 100% of us were wrong. So, I will no longer try to guess, since appearances can be deceiving.
I know the pureness of the lineage does not matter when it comes down to love for a pet, though, so I just put that out there about the test in case you just want to know for curiosities sake.