The subselect is scalar.  Thus, if you want to return more information and
can combine it into a single, scalar value, then you can still use the
subselect.

This is not tested. Just an idea.
WITH
photos_rn AS (
SELECT folderid
    , id
    , ROWNUMBER() OVER(PARTITION BY folderid ORDER BY id) rn
 FROM photos
)
,Recurse
    (rn, fname, fid, categoryid, category, preview) AS (
SELECT 0
    , F.name, F.id
    , F.categoryid
    , C.name
    , CAST('' AS VARCHAR(50))
 FROM folders    F
    , categories C
WHERE C.id = F.categoryid
UNION ALL
SELECT pre.rn + 1
    , pre.fname
    , pre.fid
    , pre.categoryid
    , pre.category
    , pre.preview || ', ' || new.id
 FROM Recurse   AS pre
    , photos_rn AS new
WHERE pre.rn < 100
  AND new.folderid = pre.fid
  AND new.rn = pre.rn + 1
)
SELECT fname, fid, categoryid, category, preview
 FROM Recurse R
WHERE rn = (SELECT MAX(rn)
              FROM Recurse RM
             WHERE RM.fid = R.fid
           )
;