Check out NativeMobileActions.SaveToPictureLibrary. This JS action has code that gets you a URL to the file/image blob. From there you can fetch the blob or pass it wherever it’s needed.
Code also here below:
if (!picture) {
return Promise.reject(new Error("Input parameter 'Picture' is required"));
}
if (!picture.inheritsFrom("System.FileDocument")) {
const entity = picture.getEntity();
return Promise.reject(new Error(`Entity ${entity} does not inherit from 'System.FileDocument'`));
}
const guid = picture.getGuid();
const changedDate = picture.get("changedDate");
const url = mx.data.getDocumentUrl(guid, changedDate);
return CameraRoll.saveToCameraRoll(url);