On the postcard creation page, when you use the "grab an image from a url option", the url required is not a url to a flickr photo page, but rather a url directly to an actual image, which can be from anywhere on the web. PS3 does not assume the image has to come from flickr).
What you need to do from the Flickr photo page is click "all sizes", then right-click the "Download the Original Size" link at the top, and select "copy link" and paste into the URL field on the postcard creation page. Soon the system will be a little smarter about recognizing flickr photo pages and work it out 'automagically'
A neater way to use your flickr images in postcard creation is to associate your flickr account (if you have one) with your user account so that when you are creating a postcard it will allow you to browse and select a photo from your flickr account directly - it's faster and more bulletproof - and it also allows other users to view and link to your flickr account generally.