Finding full file path name

Finding full file path name

Ok, here is what I want to do:
Start with a form input type file. Add an onChange behavior which will send the input value item to a javascript. The value item is a file name (a jpg) and the script replaces the src element in the html img code. This actually works fine, except the file name returned as a value is just that. What I need is a full, complete file path and name so the img src can find it anywhere on the computer.
I know that PHP can upload the file to the server using the information in the $_FILE global, but I haven't found out how it, and I, can find the full file path.
Any ideas would be very helpful.
Jim