Do you mean in image outputs or in markdown blocks?
Right now you should be able to specify it in markdown blocks using the  syntax.
In output blocks, I think we would need to modify nbconvert and you would need to add it to the output metadata in some way (possibly specific to the mimetype you are interested in displaying) and to do that you'd probably display it using the IPython.display mechanism or one of the DisplayObject subclasses classes.
If you mean the latter open an issue on nbconvert and we can try to come up with a solution either for your specific case or making it a more general feature.
Cheers,
M