$value depends on what type of field it is. If it’s an image field then you need to supply the image ID or the WP attachment ID value.
Are you using the field_key when calling update_field()?
update_field('key_1234567890123', $attachment_id, $post_id);
You said you have a lot of images, is the image field part of a repeater field? Is it some other kind of field like a gallery field?
