Yes, the URL does look correct, unless you have an Edit password on the file. When you are testing the PDF stamp make sure you are not using the Send Free Download Link option as this will not stamp the file. You should also check the, "A Few Things to Keep in Mind," section of our PDF Stamp help page:
As for the image not changing in the marketplace, make sure your browser is loading the latest images for each page, or it may keep showing you the images it already saved (cached) from your earlier visit. You can either clear your browser's cache manually before reloading the page, or hold the Ctrl key while you click the Refresh button to force the browser to reload the latest images fresh from our server.
If that still does not show you the newest images, make sure the replacement images you are uploading all fit within the size limits we describe in the image Uploader window.