Put both of them in a view like this:
|<-- center view -->|
| ________________ |
| | View | |
| |_______ ______| |
| | Image | | Label| |
| |_______| |______| |
| |________________| |
| |
Then center the view horizontal. Within the view you need to define constraints for the imageview and the label so that these define the height and width of the view.
EDIT Here are two images that show it works: