Formula Query for a related record's file attachment URL

I suggest that you m ake a formula Rich Text field field on the child record to display the image.  Then once that is working, bring that field into the parent record.

I have a few versions but here is one that will display a small thumbnail which is clickable into a larger mage.  First get it working on the child record.  Replace tghe e7 with eXXX where XXX is the field ID of the file attachment field.  This formula assumes that an image file exists (ie not a null file attachment and not like a PDF or other document type Excel / docx).

var text SmallPix = "<img src=" & URLRoot() & "up/" & Dbid() & "/a/r" & [Record ID#] & "/e7/v0" & " height=\"100\">";

var text LargePix = URLRoot() & "up/" & Dbid () & "/a/r" & [Record ID#] & "/e7/v0\"  target=_blank  height=\"2500\">";

"<a href=" & $LargePix  & $SmallPix & "</a>"