How do I assign an image to image upload field?

How do I assign an image to image upload field?

I have an image upload field in my form.  The image upload is optional - but I want to provide a default image for those cases when the user does not upload an image.  Sorta like a "no picture available" type graphic which gets stored with the record.

How can I get the form to pick up my "no pic avail" image into the image upload field?  And where would I store the image so that the script can access the image?