Whenver you’re resizing one of your images using our service we will resize it the first time on demand and after that deliver it through our CloudFront CDN for you. Depending on the amount of traffic, visits on that image path CloudFront (Amazon) will decide if they keep it in the CDN edges or if the image needs to be processed again, as the image is no longer in the CDN. This is especially true during development, once you come back after a weekend and recognize that the images might be processed again. Sadly we can not change that behavior from our side as we’re using CloudFront, and CloudFront itself determines if it keeps entries in their cache or not. The good thing, if you’ve launched your or your clients website and images will get visited more often, CloudFront usually keeps your images and the speed is as good as a static file would be.
You can see if it Hit or Miss CloudFront in the network tab of your browser, by inspecting the network request and look for a header that either contains Miss or Hit from CloudFront.