How to add image to only front page of Drupal 7?
Here is another approach:


create "a" node (pick whatever content type you want), say it's node id is 123 (generated automatically by Drupal when you save it). Or add a "path" to it like my_page_with_image.
include that image in that node and make sure your input filter that you use (i.e if you use "filtered HTML", which is what I'd recommend) accepts "images".
set node/123 as your frontpage (or my_page_with_image instead, if you added a path for it).


Next question? Maybe "How do I set that page as my frontpage"? ... Ask (via a new question, only 1 question per 'question' here ...) and pretty sure you'll get the answer to that question to ...

PS: What is so special about all your pages where you are considering using PHP for it's actual content? You are aware that using PHP for those things is in general NOT recommended, right? And that you have to be very carefull in which user "roles" will be allowed to create such pages ...