The purpose of this feature is to support thumbnail requests from secure (SSL/HTTPS) pages that would otherwise show an "insecure elements" message to your visitors when calling an image as HTTP.
To use this feature, you simply call the image using img src as usual but you replace http with https and the image will show without causing your page to display errors about insecure 'elements.'
This feature is provided at no charge and is available to all users.
If your website is secured with HTTPS but you fail to use HTTPS to make requests from ShrinkTheWeb, then your visitors may not see the screenshots due to security mismatch. In this case, you need to ensure that all accesses to ShrinkTheWeb use HTTPS instead of just HTTP.