There are two ways you can get the cache cleared for the assets with AEM DM Hybrid setup:
1. In AEM you can change the Default Manifest setting for the image server. You can fine-tune quality (JPEG quality, resolution, resampling mode), caching (expiration), and prevent the rendering of images that are too large (defaultpix, defaultthumbpix, maxpix). More detailed information can be found here
2. Open a support ticket along with the list of asset URLs and will get that processed from the backend.