Insert banner into all posts (page.php)

as the name suggests, page.php is for pages;
for posts try index.php or single.php;

the html code for an image is:

<img src="../uploads/banner.png" alt="" />

you will possibly need to use the absolute path to the image.

index.php:

[Code moderated as per the Forum Rules. Please use the pastebin]

Where to add the image tag?

this is my Single.php

[Code moderated as per the Forum Rules. Please use the pastebin]

I want to add a picture to all posts.

Where to add the image tag?

you need to describe in more detail where you want the image to show – then someone might be able to point to the location in your template where to add the image code.