Hmm, it's possible that there's a site problem/issue on our end that's causing the pic upload problem but I'll proceed with the information you've supplied.
When a goldfish's mouth is stuck in an open position it is either being caused by something that's enlodged in it's mouth, i.e., piece of decor, gravel,etc.etc, or unfavorable water conditions, i.e.,presence of ammonia and/or nitrites in the water.
From what you'e described there existed unfavorable water parameters due to the fact that no water change was done for 8 days but you have since rectified that although you did mention that the fish still appears stressed.
Unless you feel that it's possible that the fish might have attempted to eat something and therefore has something enlodged in its mouth I'm going to treat this as a water quality issue.
First thing to do is ascertain that the water temperature falls within the correct range for goldfish which is 62f-72f degrees. If not then slowly adjust it.
Next--do a water change, making sure to use same temperature water, but instead of changing all the water change only 1/2 of it as sometimes changing too much water will shock a fish due to a drastic change in water parameters, i.e., ph, temperature. etc.,etc.
I know this is a no-brainer but I always like to reinforce this and that is to make sure to add a dechlorinator to the new water before adding to the bowl/tank. Keep in mind that chlorine is lethal to fish and chloramine is turbo-lethal to fish.
The dechlorinator I recommend be used is called AmQuel +.
AmQuel + will detoxify ammonia, nitrite, nitrate, chlorine and chloramine in the fish's water. With the addition of AmQuel+ to the water you will be guaranteed, at least temporarily, ideal water conditions for the fish which will rule out any water quality problems in case the open-mouth syndrome continues which I don't think it will. Click on link below for a view of the product which is easily found at most major pet stores.
Also add 1 teaspoon of aquarium salt to the water with the water change.
Continue doing the above listed regimen every other day for the next 2 weeks adding the corresponding amount of Amquel+ and aquarium salt. As you'll be changing 50% of the water you'll only need to add 1/2 teaspoon of aquarium salt wth each water change.
Make sure that the water temperature does not fluctuate too much and also important is to not overfeed the fish. Feed only a very small pinch of food once a day as overfeeding tends to deteriorate water quality rather quickly especially in small systems such as yours.
If these recommendation do not effect a cure then we must consider the possibility that the fish has something enlodged in its mouth for which you might need to take a hands on approach but we'll wait on that.
Good luck and let me know if you have any questions.