I still don't think it's impossible.
Again, evolution doesn't occur because there's a 'need' for something. It happens because there's a change in the genetic structure.
If an organism survives and passes on new traits, that's what happens. It's not even necessary for the organism without the new traits to be replaced.
Sure, it's a longshot for photosynthesis or something analogous to occur in an animal's physiology, and it's even more of a longshot for this to continue to the point of an animal becoming completely dependent on photosynthesis, but it's still possible. (Even in the case of plants, they still require the uptake of nutrients, so it's not a matter of an animal becoming independent of any material intake).
I know I'm simplifying here, but here's a bit of an outline of how I see that it could happen -
Imagine a series of changes do take place where an animal produces cells that manufacture compounds that its physiology already has a use for, that previous incarnations needed to ingest in order to acquire. These cells wouldn't even necessarily be in their skin, they could be blood-born.
These changes aren't necessarily, and most likely won't be, a single generation manifestation of photosynthesis. The process may even be happening already. The precursors to photosynthetic structures would just be something the organism happens to produce, but doesn't have an immediate use for. As long as they aren't directly poisoning the organism, or wasting too much of its energy on unused structures and in turn causing the organism's demise, they would continue to manifest in future generations.
Eventually, a photosynthetic structure would come about. Or not. But if it does, the animal lives with it, benefits from it, and passes on yet another tiny tool to improve the chances of its offspring's survival.
At this point, it would only be a supplemental photosynthesis. Pure dependence (this, I don't think would happen in nature) would require a lot more to change, and even if this did happen there would still be a need for the organism to acquire raw material.
To move closer to pure dependence on photosynthesis, the animal would have to use very little energy. It takes days of photosynthesis for a plant to gather enough energy for an animal to survive on for a few minutes or hours.
The animal would also have to be less massive than its ancestors, again due to the overwhelming energy constraints.
It would also most likely be very docile, as well as exclusively diurnal.
For these reasons, sure, I don't think an animal would ever come out of nature to be completely photosynthetic, but there's still always a possibility.
I do think it's a much stronger possibility that a supplemental photosynthesis can occur in animals, however.