I was previously using JA Amazon S3 version 2.5.6 and the images for Simple Image Gallery Pro galleries were uploaded to S3 and the links correctly replaced by the JA Amazon S3 extension. I now upgraded JA Amazon S3 extension to version 2.5.9 and now the extension is no longer replacing the links for SIG Pro image galleries. The extension does correctly replace the links for individual images, but not for galleries. I updated SIG Pro to the latest version 3.7.0 with no change in results.
I included a link to an example K2 item with an image gallery. You can see that the individual image is coming from S3, but the gallery images are loading locally instead of from S3.
