I believe it is still possible to update images even that there is no custom field for this. If you open the the listing template image tab you can use "reload images from product" option and update the listing then new images will be sent to the channel. You can do the same in a bulk as well. Just open up the configurator select multiple listings press the gear icon on the top right > Reload images from selected templates > click process. All the new information will be sent to the channel.
If I've created products on Bigcommerce through linnworks, and I update the images for some reason, it does not allow me to revise the images or to update them from within linnworks.