The default thumbnail image is located at templates/images_default/nothumbnail.png (change images_default to images_yourthemename for other themes)... when using website thumbshots, there's also a no thumbshot image at templates/images_default/nothumshot.png. The deftault version of these is a completely transparent PNG file. You can replace the image with any other image you like.
For the WSN 8.0 and earlier series, the same effect can be accomplished via template edits. Go to Admin Panel -> Themes -> Manage Templates and click the topic bit template. Find this section:
<IF {SWITCH_THUMBSHOTS} or {SWITCH_PRICES} or {LINKNUMIMAGES}>
<div class="floatleft linkboxleft">
<IF {LINKIMAGEBOOL}><div class="thumbshot"><a href="{LINKDETAILSURL}"><IF {TID} contains main.tpl>{LINKIMAGE[180 <,> 160]}<ELSE>{LINKIMAGE[120 <,> 90]}</IF></a></div></IF>
</div>
</IF>
<div class="floatleft linkboxleft">
<IF {LINKIMAGEBOOL}><div class="thumbshot"><a href="{LINKDETAILSURL}"><IF {TID} contains main.tpl>{LINKIMAGE[180 <,> 160]}<ELSE>{LINKIMAGE[120 <,> 90]}</IF></a></div></IF>
</div>
</IF>
Change that to
<div class="floatleft linkboxleft">
<div class="thumbshot"><a href="{LINKDETAILSURL}"><IF {TID} contains main.tpl><IF not {LINKIMAGEBOOL}><img src="{IMAGESURL}/nothumbnail.png" style="width: 180px; height: 160px;"><ELSE>{LINKIMAGE[180 <,> 160]}</IF><ELSE><IF not {LINKIMAGEBOOL}><img src="{IMAGESURL}/nothumbnail.png" style="width: 120px; height: 90px;"><ELSE>{LINKIMAGE[120 <,> 90]}</IF></IF></a></div>
</div>
<div class="thumbshot"><a href="{LINKDETAILSURL}"><IF {TID} contains main.tpl><IF not {LINKIMAGEBOOL}><img src="{IMAGESURL}/nothumbnail.png" style="width: 180px; height: 160px;"><ELSE>{LINKIMAGE[180 <,> 160]}</IF><ELSE><IF not {LINKIMAGEBOOL}><img src="{IMAGESURL}/nothumbnail.png" style="width: 120px; height: 90px;"><ELSE>{LINKIMAGE[120 <,> 90]}</IF></IF></a></div>
</div>
Save changes and you will now see the nothumbnail.png next to topics that don't have any images.