The removal and exchange of bad images should be one of the first steps in your processing workflow. It is intended to be done once but, however, during the processing it may happen several times that you have to throw out an image which you considered all right in a previous step.
A bad image is one where you have artefacts which cannot be removed by any software. Frequent examples are:
Possibility 1: replacing it by the average of the surrounding images, which can be done using timelessAverageEDF.
Possibility 2: replacing it by an empty image, which can be done with timelessCreateEmptyImage.