Hi John,
You can create a new field for the image in leads and then add the images (as binary content) in the import sheet (ideally as CSV). Below are the steps which might be helpful for you.
Create new field:
Activate the Developer mode from Setting-> Activate Developer Mode
Go to the Leads module and edit any record
You will see a Bug icon on the right side of the Header menu
By clicking it, a list will be opened. Click the 'View Fields' option.
Create a new field like 'x_image_1920' and the field type will be 'binary'. It should have 'Stored', 'Copied', and 'Blacklisted' checkboxes as checked.
Copy the Field Name and then Save the field.
Again, click on the Bug icon and click on the 'Edit View Form' option.
XML view will be opened. Add the following line after the two widgets and before '
'.
Save the view and refresh your page.
Now, you'll be able to add images to Leads.
Import the Data:
Download sample data from leads and include the image field in it.
Check how it created binary. You need to add binary of your images the same way and then import it.
Please note that the side of the file will be increased and you might need to update the config of odoo/server or import the files in small parts.
Import all leads then make a script to import images to the related leads.