set a default thumbnail for a gallery

Hi folks,

I have to different gallery in to the user profile. For the first one I use the thumbnail I got by cropping and/or scaling the original image. For the second I would like to set as thumbnail a default image, so users cannot see a preview but they just know that there's the image and if they want to see it they have to click on it.
Any idea of how to manage that?
Thanks a lot!