No, you can’t upload a sent card after registration by the receiver.
As mentioned, you could ask, maybe suggest to send the picture you already have, maybe via a download link avoiding the need for email adress.
If there is no reaction, you have to make your peace with it.
(There is purpose, afaik, as to avoid conflicts both of software and of interests (not giving the sender the power to influence a recipient’s wall))
Thanks for responding everyone.
I am ok with the cards not being on the wall - no need to bother people. I was just curious.
Also thanks for merging the posts. I did use the forum search, but couldn’t find the topic.
Hello Everyone
I am new to postcrossing. I have sent out many postcards and am starting to get some in return. I have noticed once I registered them the postcard shows up on my page. How do I do that when sending out my postcards? Thank you!
Look - here is the answer of your question:
Good success with uploading your pictures.
BTW, I always recommend to new members to read Help and F.A.Q. - you can learn a lot about postcrossing if you read all themes there.
Thank you for your help
Don’t think I uploaded my first 5 either 
There is one problem though. I always upload the image of the card that I have sent. But a few recipients delete it. And there is nothing you can do about it.
Maybe the receivers will sometime. I don’t upload received cards right away, but do it in batches. I did noticed I haven’t uploaded all my cards of 2015, so there is still hope. 
Also waaaaaay back, when postcrossing was new, uploading pictures was not available yet. So if you miss a few, you just fit in perfectly with the members from that era! 