Signing your images is a good first step toward avoiding malicious use—it keeps them from being used to produce unwanted renders or as a CDN for someone else’s site. We recommend that all images be signed, and require it for Web Proxy Sources.
In general, we recommend that you sign URLs using one of our Client Libraries; this makes the process more efficent and less prone to error. However, we also recognize that you may need to sign images individually or in small batches sometimes, and we've upgraded the signing tool in the Dashboard to make that easier. You can now sign up to 100 images at once, and the tool checks to make sure your Source is set up for signing.