[R] downsize-images: use find instead of ls
[R] downsize-images: use find instead of ls

Because find wouldn't error out if one extension wasn't found.

Signed-off-by: Yan Han Pang <2a3dbd0474bc7e70adfd773b05f6c3249124ff3d@users.noreply.github.com>

yanhan/bin-src,yanhan/bin-src,yanhan/bin-src,yanhan/bin-src