You probably have Drag'n'Drop-ed Picture or File Attachment (base64) data field from outside of group of repeating section
- for example, in my screenshot drag'n'drop-ed Attach1 on repeating section control
or, what is the same (creation of a data field outside of repeating structure by inserting independent data field) by drag-n-drop from ribbon.
You should inserted File Attachment data field under repeating section data control in Fields pane
- in my screenshot File_Attach_RepSec under group2
and then DnD-ed it onto design surface.
Or simply create a repeating data field of type Picture or File Attachment (base64) like aaa in my screenshot and then drag'n'drop it onto design surface having chosen Repeating Section at 1st DnD and File Attachment at second one