What is "to cradle a picture"

Cradle \Cra"dle\, v. t. [imp. & p. p. Cradled (-d'ld); p. pr. & vb. n. Cradling (-dl?ng).]

1. To lay to rest, or rock, as in a cradle; to lull or quiet, as by rocking.

   It cradles their fears to sleep.
   --D. A. Clark.

2. To nurse or train in infancy.

   He that hath been cradled in majesty will not leave the throne to play with beggars.
   --Glanvill.

3. To cut and lay with a cradle, as grain.

4. To transport a vessel by means of a cradle.

   In Lombardy . . . boats are cradled and transported over the grade.
   --Knight.

   To cradle a picture, to put ribs across the back of a picture, to prevent the panels from warping.