Hello HeyMoe -
Unlike our stock templates where you can adjust the number of images on a listing by listing basis, you must set your custom templates to have the appropriate number of images. Note that you can add additional image spaces to the custom template and only the utilized image spaces will display, so you can set your template to have more images than you may usually use for instances where you wish to add other images to your listing.
To make changes to your custom template, hover your cursor over the Listings tab and select Manage from the Custom Template area and then select the template you wish to use. You will need to add the appropriate image tags from the menu on the left to your custom template where you want the images to show in the template and save the changes. Then you can apply this template to your listings and select the images you wish to display.
If you run into any difficulty or have further questions regarding this, please feel free to file a support case from under the Help tab of the site and we'll be happy to assist you further.