Do you need permission from the photographers of images made into dingbats even if you intend them to be a free download for personal use only?

Nope. A dingbat is a new creation.

Even if it's a new creation, it's based on someone's work, it's always better to get the permission. Check if there is a license with the photo (copyright, creative commons, copy left...) If there is not any, try to contact the author and explain your project.