When you upload an image through a form your html will contain a line such as
<input type='file' name='img1' ... >
When the image is uploaded to the server it sits in a temporary file which is set in your php.ini file and the name of the file is stored in the variable $image_name.
So if your uploaded image has the name picture.gif then on the server $image_name will correspond to picture.gif
Hope this helps
Paul