If you uploaded an image of a postcard that you sent and now it is gone, the most likely reason is that the image was deleted by the receiver. Control over the image displayed mimics real life - whoever has the physical postcard can choose if they want to display it online. So, once a member has received their postcard, they have control of adding/modifying/deleting the image corresponding to that Postcard ID.

If the image that is missing is of a postcard that you received, then there may be a technical problem. Make sure that you are able to view other postcard images on the Postcrossing.com site. Try to upload the image again and see if the upload completes successfully. Lastly, Postcrossing administration may remove images that they deem to be unacceptable.