... the "php elements" in the templates are tags like {tag_name} ...
... so you can find in home.html tags like ...
{random_image} : for the random image
{if categories}{lang_categories}{categories}{endif categories} : for the categories
{new_images} : for the new images
... a.s.o. ... you will find quickly the meaning of the tags ...
... all other code in home.html is "html" and for your website design ...