Hi @nickolaas
To display an image size other than ‘full’, you need to firstly set the return type to either ID or Object.
From this data, you can then load the correct image size.
Please see the image field docs for examples:
http://www.advancedcustomfields.com/resources/field-types/image/
Please note you are using a sub field, so load the iD or Object via get_sub_field
Thanks
E