You can use either the CMIS API or the Public REST API to list renditions. Here is the Public REST API swagger docs on renditions: https://api-explorer.alfresco.com/api-explorer/#!/renditions/listRenditions
Once you have the node reference of the rendition you want, you can refer to it similar to any other node reference. Here is a link that downloads the image from Alfresco:
Here is the same link, but this one includes arguments for that cause the rendition to be queued for creation if it does not already exist (queue) and one that returns the placeholder image if the rendition does not already exist (ph).
If instead you want to download the image using the user's Share session, you would do so via the Share proxy, like:
Hope that helps.