Hi robin
Thanks for the answer.
But in fact, I have vector files. I want to paste the related png format to the uploaded vector file.
I go to edit product and add related png files manually (while I checked unlettered in its license) and save and then update the product.
But when I download png file, it is a jpeg file.
I finally found a trick myself: I deactivate symbiostock premium first then upload png file to its related product and finally activate the plugin again.
But I am looking forward to your proper guide.
Thanks again