Quote from Mike_191084 on May 8^th, 2012, 4:52pm:

 
You can upload more images immediately.  And you don't have to keep storing your images after your cards are submitted, unless you plan to use the same images again.  I usually delete my images shortly after submitting them as cards.
 
Hope this helps!
 
Penny

To further clarify - after you upload and create your cards, there will be a review process which can be up to 2-weeks before they will show up in your store.
 
So, as the ladies have said, you can continue to create every day and before you know it your store will begin to populate!
 
Be sure and review the great video tutorials in the wiki and the submission guidelines, both can save you save time in the long run.
 
Good luck!