If you add the image to the top of the blog in your admin. Then click the "source" button and add:
style="display:none;"
to the image tag so it looks like:
<img style="display:none;"
Then that image will still show in the results area as the small image but won't be displayed in the blog itself.
If you need help you can attach an image you want to use and I can do it for one so you can see what I mean.