This is going to sound harsh, but if three vets, who have actually examined your dog, haven't been able to help, I'm not sure what a bunch of mostly non-medical strangers on an Internet forum will be able to tell from a photo.
We'll leave this thread open for a bit, but be aware that diagnostic guesses are a serious violation of forum rules. This is for the protection of both your dog and, because of liability concerns, the forum.
Probably the best we can hope for is a suggestion about what sort of specialist to seek out, but that suggestion should really have come from your vet as well.
When we were faced with a mysterious array of health issues with our miniature schnauzer, our vet offered three different options for specialists to consider, despite our complete confidence in her experience and expertise.