Well, first of all, there is no direct relation between a blueprint and a template. The blueprints are only relevant if you use the panel but you don’t need the panel to be able to use Kirby because you could just create the text files in any text editor.
So basically, a blueprint defines what a user can write into a text file and the template then pulls that information from that text file.
If you want the user to be able to add some images for a gallery via the panel, you would need to define a field in your blueprint that lets the user enter a list of images. One way of doing that would be a multi-select field (e.g. using the visual select plugin: https://github.com/storypioneers/kirby-selector), another a structure field (http://getkirby.com/docs/cheatsheet/panel-fields/structure).
In the template you would then pull that information from the text file via php, depending on what sort of field you have created.
If you use the selector field, for example, you would create a new field in your blueprint:
gallery:
label: Images
type: selector
mode: multiple
types:
- image
which would let the user select images from all the available images in the page folder:
In your text file, you would then end up with a comma separated list of images:
Gallery: 758px-sydney_skyline_at_dusk_-_dec_2008.jpg,forrest.jpg,green.jpg
Now, to pull that information in your template, you could use the following code:
<?php
$gallery = $page->gallery()->split(); //splits the comma separated list so you end up with an array that you can iterate through
foreach($gallery as $galleryImage) : ?>
<img src="<?php echo $page->images()->find($galleryImage)->url() ?>" />
<?php endforeach ?>
So much for the basics, hope it helps. You could of course first check if there are images selected and so forth …