This simplest way to accomplish this is with a template edit, or two.
As you know at what trophy point value a new star will be shown, try the following. I'm using the message_user_info template as an example, as you may want it displayed elsewhere. Replace the numbers I used with the actual trophy points required (-1).
Find: <xen:require css="message_user_info.css" /> and below that add:
HTML:
<xen:set var="$starWidth">
<xen:if is="{$user.trophy_points} > 19">
oneStar
<xen:elseif is={$user.trophy_points} > 49" />
twoStars
<xen:elseif is={$user.trophy_points} > 99" />
threeStars
<xen:elseif is={$user.trophy_points} > 149" />
fourStars
<xen:else />
fiveStars
</xen:if>
</xen:set>
Depending where you want the stars to show, add this (in this example below the h3 that contains the user title)
<div class="trophyPointStars {$starWidth}"></div>
Add to your EXTRA.css template, something like this:
HTML:
.trophyPointStars
{
background: url('@imagePath/to_your/stars_pic.png') no-repeat;
height: 24px; // replace with actual height of the pic
}
.oneStar
{
width: 24px; // actual width of a star
}
.twoStars
{
width: 48px;
}
.threeStars
{
// you get the idea, :)
}
I never tested the above, but it should work.
BTW: from your first message it is clear that you were talking about trophy points, and not user banners. Just to let you know,