The best way is to have a field in Marketo that stores the picture URL. When someone registers or is registered to the user, you will have to push this URL to Marketo together with the lead info. This can be done with a hidden field in a Marketo form and some scripting, or with a hidden form and some scripting.
Here is how to make the second solution work :Make a Marketo Form Submission in the background
we're assuming the photos are stored in a separate database; Marketo's not a good place to store these.