Image in Simple Page

I want to put a photo in one of my simple pages. I'm not sure where to place it in the code, since the plugin's show.php is for all simple pages, not just the specific page this photo is for.

You can edit the Simple Page itself, within the administrative interface:

If the image you want to insert is not an item, you can create an images folder in the theme and upload the file there, and call the file to the page.

Would I call the file to the page on the form in the administrative interface?

Yes, upload the image to the image directory and just call it in the page in the admin interface:

<img src="<?php echo img('image.jpg'); ?>" />