When a node is designated to be a 'template base' for the subtree, all newly created descendant nodes will contain the template article text and template images by default.
Currently the template resides inside the article of the node which is the template base (at the most fundamental node of the sub-tree).
The page 'general' inside the Node properties dialog contains a checkbox 'Is template base' where one can make any node a 'template base' (of its own subtree). Inserting a new node inside a template-controlled subtree will result in showing the 'new node' dialog (unless you have disabled this in the settings screen):
This dialog presents you with the option of inserting a node using the current template, or a blank node (ignoring the template).
A template node can be distinguished (from regular nodes) by a little cross-mark which is visible on the left-hand side of the node-icon:
You can disable⁄enable the 'Insert new node' dialog in the settings screen, category 'template options'. There you will find ae checkbox titled 'When adding a node show the insert dialog'. If this option is enabled, then inserting a node inside a template-controlled subtree will result in showing the 'New node' dialog. When this option is not enabled, inserting a node inside a template-controlled subtree will always result in inserting a new node using the current template.