Questions

To Change / Replace an Image:

To replace an image, simply upload a new image with the same filename, watch out for typing errors:
NUZ > Organize > Files > +1

To Delete an Image:

Each image file has the xDelete function in the list on Organize > Files and also when you are viewing the image ( you will get a warning before the file is deleted ).
However, when an image has a database entry, ie. it has been published on Netcells or on your site, you should, first of all, set it to Off, before deleting the image file in Organize > Files.
You do that in: Publish > Images > click the Image > Edit - - -> set ShowWhere to Off
Once you have done that and you no longer need that Image you can simply overwrite its database entry so that it corresponds to a new Image (that Image should be already uploaded as described above).