There sure doesn't seem to be any easy way to change the default icon, but it's pretty easy to bulk-edit your contacts and add the same image to every entry. As long as there's an image, the creepy guy won't appear. You'll need a good plain-text editor to do this, and Android File Transfer if you're on a Mac.
What I did was edit a single contact and give it the image I wanted to use for all. Then I exported the contacts, copied the resulting .vcf file to my computer, and edited it with a plain-text editor. It's pretty easy to see which glop of text is the encoded photo. Just copy and paste the same glop to the contacts without photos. Be very careful to preserve the formatting -- spaces and blank lines -- or the copied data will be ignored. If you have hundreds of contacts, use your editor's macro or repeat function. Save the edited file, copy it back to the device, and import it back into Contacts. (I actually deleted all my contacts before importing, but I don't know if that's required.)
This doesn't help you going forward, because new contacts will always get the default image until you add a pic. So now every time I add a new contact I just take a picture of my shoe or something.