Thought I would share, I had the default size thumbs enabled and then changed the size and needed to rebuilt but no function exists built into the admin to resize all images.
(note this is only for images and galleries)
Code:
#!/bin/bash
# batch resize mechbunny thumbs
for img in */*.jpg; do convert -adaptive-resize 160x200^ -gravity Center -extent 160x200 "$img" "${img%/*}/thumbs/${img##*/}"; done
to be run inside /media/galleries folder.