You can do that by tweaking one of the text labels.
From SYSTEM OPTIONS => Configure Text Labels, look for the "Clips Popup: Choose Clipart" entry.
That's what appear after the " => " in the cliparts popup (IE "UPLOAD A FILE => CHOOSE A CLIPART")
If you try to replace that textlabel with something like:
CHOOSE A CLIPART
<br><center><span style='font-size:12px;'>
Click the thumbnail to add the clip to your design
</span></center><br>
You will obtain a descriptive text on top, which will be shown for both normal and user uploaded cliparts.
Hope this helps, let me know!
Alex