avoiding broken thumbnails if no image was uploaded?

Hello together,
I am playing around with phprunner.

Now I have managed to upload images into the database.

But if a users do not upload an image, in the list page I'll get the "broken image" view from the browser. This makes sense, as there is no data in the table. but this is ugly <img src='https://asprunner.com/forums/file.php?topicimage=1&fieldname=question&id=17036&image=1&table=forumtopics' class='bbc_emoticon' alt=':)' />
Is there any solution to avoid this and/or show a default image if no picture was uploaded?
thanks,

Dani