Thank you, Stephanie, for the helpful tips. I always have enjoyed seeing "Before and After" pictures of all kinds. Your subtle changes made a big difference. Congratulations on the sales!  
It would be fun to see what other artists have done to improve their existing cards, too.
On a related note, I posted a GCU Support question in the Forum on August 25th concerning one of my updated images that had an immediate sale. I have yet to receive a single reply, although I have had 1,112 people read the post. If you, or anyone in GCU Support, could help me find an answer to my question, it would be much appreciated. Thanks!
(Here's my original post of August 25, 2017
 
After a customer orders a card, how long does it take for the Artist Services notice of a sale to appear in our email? I'm wondering this, because last night at about 10:00pm, I changed an old card by updating the text with a smaller, different font. It was a seascape/painting of mine in which I also changed a seagull to make it sharper and much nicer.  
It was a needed improvement, in my opinion. This morning about 12 hours after I made my changes to the image, I received a notice that 2 of these cards have been sold.  
So now, I'm wondering if what the customer saw on the GCU site is what they will receive. I would not want someone to get something different than what they originally ordered even if it's a much better, updated design.