....although that brings up the question of
WHY you want an image to be on it's own page??
If it truly needs to be all by itself on the page then I would recommend wrapping it in an svg...that solves
most of the image issues.
You can even give Sigil a template file that will do this for you when you select TOOLS/Add Cover. It automagically adds the proper image
width/
height/
filename
Just add the
COMPLETE xhtml file to your Sigil Preferences Location (EDIT/PREFERENCES/Open Preferences Location) and name it "cover.xhtml"
Here is the one I use... you can modify it however you wish:
Edit: I almost forgot to mention CalibUser's Sigil plugin:
InsertImageSVG