Well, you'd probably need a custom plugin to do what you want normally. If you use the image as a background, you can do it by changing the size of the container. For example:
<if condition="$postsbarwidth=$post['posts']/10">
<div style="background-image:url(images/myrepeatingimage.gif);background-repeat:repeat-x;height:15px;width:{$postsbarwidth}px;padding:0px ;"><img src="$vboptions[cleargifurl]" alt="" height="15" width="$postsbarwidth" /></div>
</if>
This will show 1 px of width for every 10 posts for a images/myrepeatingimage.gif up to 15 px high. If you want to show full images, you can upload the attached plugin to allow use of intval, then change the conditional to something like:
<if condition="$postsbarwidth=intval($post['posts']/10)*15">where 15 is the width of the image.
I'm sorry, I'm affraid that this is pretty complicated...