Prior to the task fMRI a staff member explained how it would work to participants. Here a participant is being shown a triplet of shapes (on the obverse side of the paper being held).

This resource can be downloaded or viewed using the link: mri_triplet2.jpg

If you have wget available (typically on linux systems), then you can also obtain a copy using the command

 wget  -nd  biobank.ndph.ox.ac.uk/ukb/ukb/images/mri_triplet2.jpg