Setting an default image in part master creation?

Hi expert, Would you help one thing? I try to set default image when new part is created or existing part is saved without an selected image. So, I created server side code(C#) as below. this.setProperty("thumbnail", "<thumbnail>.././customer/images/LED.jpg</thumbnail>"); When I checked it in database, it is stored in DB. But, when I refresh that part, no image is appeared. I also tried the code below. this.setProperty("thumbnail", "vault:///?fileId=B5F8B4BAEB3343459C758672DA727C5J"); But, it also didn't work. Would you help on this problem? Thanks.