Welcome to MR.
I have seen these 'spacers' used
Using CONVERSION:
Remove Image by size? No.
Is this ONE BOOK(title)???
Using the EDITOR. Probably Easily.
In the Editor: Tools: Reports: Images (sidebar)
There you can see thumbnails, how many times used, size
If you are real lucky,

the same image will be used everywhere.
Double click, and it should take you to a place where it is used.
Highlight the code: ctrl-F (Find), Replace is blank
eg
Code:
<p class="copy1">
<img alt="colophon" src="../Images/colophon.jpg"/>
</p>
A 'dirty' way, that might get the job done (or not), is to simply delete the image file (Note: the preview does not actually lose the image until the book is saved (7.12 Win. I switch off of and then back to the page)