I don't think this is documented on the wiki. The following text explains how, but could be improved.
If you have not already uploaded the images, but you have uploaded the files. You can find each file, click the “eye” icon to see its metadata. Then scroll down to the thumbnails section. Click ‘add’ and you can add and upload thumbnails online.
If you have already uploaded the thumbnail images, then you should go to the dataset and look for the thumbnails section. You’ll need to click the ‘edit’ button on the right had side of the table header to switch into a table view of the thumbnails. Then for each thumbnail you want to link to a file, click its “edit” icon. You’ll get its metadata form. Find the “Thubmanil of” field. Click that and select the file it should be linked with.
I don't think this is documented on the wiki. The following text explains how, but could be improved.