…size

For now, saveImage writes data to image.ckpt via an append FileOutputStream,
when there is a non-zero size file named image.ckpt, a disaster would happen
due to a corrupt image file. Even worse, fe only keeps the lastest image file
and removes others.

BTW, image file should be synced to disk.

It is dangerous to only keep the latest image file, because an image file is
validated when generating the next image file. Then we keep an non validated
image file but remove validated ones. So I will issue a pr which keeps at least
2 image file.