Allowing registrants to upload a sponsor logo when registering is controlled at the individual registration item. First navigate to Registration and then Items.
Allowing a Registrant to Upload Sponsor Logo
Find the registration item that when registered for, allows the registrant to upload a sponsor logo.
Click the pencil icon next to the item.
In the General tab under Options, locate the Allow Registrant to Upload a Logo toggle and enable it.
Click Update Item to save.
Now when a registrant goes to register for the item you just updated, they will be prompted to upload a sponsor logo.
If you want to download the logos registrants have uploaded check out this article.