If you want the image on the tab itself instead of the content area, something like this perhaps...
<tabitem>
<tabitem.headertemplate>
<datatemplate>
<stackpanel orientation="Horizontal">
<image source="Images/myimage.jpg" width="25" height="25" stretch="Fill" />
<textblock text="Header Text" margin="5,0,0,0" />
</stackpanel>
</datatemplate>
</tabitem.headertemplate>
<!-- put tab content here ! -->
</tabitem>