How do I show an image on a page, after user uploads it?

There are several ways to do this, depending on what you actually want to do.

If you want to display all images in the selected folder, you need to use PHP file system functions to loop through the folder, identify all the images, and store them in an array. Then loop through the array to display the images. If your server supports PHP 5, you can use scandir() to examine the contents of the folder. If you're still on PHP 4, the scandir() page in the PHP manual explains how to use opendir() as an alternative.

If you want to display only the image that has just been uploaded, use a session variable to capture the file name, and use that to display the image.

The other way is to store details of the images and who they belong to in a database, and use that to display the images you want.

Whichever approach you adopt, you need to be careful that the images being displayed on your site are not obscene or stolen from other sources. Allowing others to upload files to your server opens you to a lot of security risks that need to be taken into consideration when coding.