A Gravit design question – inserting saved designs into print on demand websites

After much trial and error I have chosen to use Gravit designer to make up my designs for print on demand websites . There is the though something I am not sure how to go about and I have tried to put it in Google with no luck and so hopefully someone out there can help me.

My dilemma is - if I save a design on my computer from Gravit not only will I not open when I click on it but how can I insert it into a print on demand website in order to sell it?.