a) If you want to add multiple files to the current field with existing pictures - you would need to have first Retrieve a Record module and then in the Update a Record module - in the attachment field - change to “map” and in the filed use following formula:
add('image array from Retrieve Record module' ; 'new image url')
b) if you want to add multiple pictures and the field doesn’t have anything there yet. You can just use Update Module and insert array of URL (+name) using an aggregator.
You can also add multiple pictures in new rows like in the video example below and then use a Roll-up to show all of them in a single field.