- tests (for questions of the "file upload" type)
- in their profile (provided the "extended" profile is activated
Where do you want them to use images?
What you're looking for, in my opinion, is an additional field (select "file upload" as the field type).
If they self-register, they can add it upon registration.
If they are supposed to upload it later, provided that "extended profile" is activated, they can edit their profile and upload their additional certificate.