you need to have a file field for each of the 3 images, and the pdf, so 4 file fields and a file upload behavior for each.
on the update page, each file field will need a corresponding hidden form element.
the hidden form element will have the initial value bound to the corresponding column from the recordset used to populate the update page. and in the file upload behavior, bind the Default File Name setting to the hidden form element.
this is covered in the tutorial I linked to previously: