General Question Of Before Image Vs Checkpoint

I read recently that: "if a before-image was made before an
interruption in database processing, you use the before-image dump &
simply begin processing again from just before the last transaction.
Any checkpoints are irrelevant."

I'm not a dba here, so bear with me... What I don't understand is, why
are checkpoints irrelevant when you did a before image dump? Why not
begin from the last checkpoint? Probably part of my problem is
understanding the difference between the before image dump and a
checkpoint. I know that the checkpoint is a db even that suspends
processing & writes data to the disk. It represents a moment in time
when everything is OK in the db. That sounds a lot like a before image
dump :) Maybe the checkpoint is further back in time (?)

Basically, if there's an interruption and you need to restore, do you
resume from the transaction just prior to the last one, from the before
image? Or do you resume from the transaction from the checkpoint...

thanks for any help -- jason shohet