Normally Sigil and/or Calibre questions would be asked in their respective forum.
However, to delete images simply highlight the image(s) on the left side of the screen (Bookbrowser in Sigil) and hit the delete key.
You will probably also want to delete the code which references the image from your html file(s). That can be done with a regex:
search: <img.*?/>
replace:
nothing/blank