We use nifty images, except for an 3D array the image also contains voxel spacing, and origin and direction in a world frame

Yep, make sense ... you can just upload them as debug samples from local files.
I guess the main difference is the context, debug samples (used for debugging) vs artifacts (might be useful from other Tasks / context)
https://github.com/allegroai/clearml/blob/6b9297660e0ed83a77bce3da2fab384c552206fd/examples/reporting/image_reporting.py#L36