The problem's in two parts, the first of which you seem to have solved.
a) Get Arduino to read tag, send tag's ID out of Arduino over serial port
b) Do something with that number in an attached PC
The material at...
... goes into the first problem. While getting the ID "out" to the serial monitor is a good start, it probably isn't the best "way to go" in the long run. (It is a GREAT start, actually... means you've overcome the not-trivial hardware issues... even though I am going to suggest what, compared to them, is a small modification: use a "dedicated" second serial port at the Arduino, set up with NewSoftSerial, to avoid various problems.
The second part requires that you know how to write programs for the attached PC. The challenges in the proposed application are minimal. Any decent general purpose language would be able to handle the job. The most difficult element would be accessing the stream of data coming in on the serial port... never trivial, but not "rocket science".
You could define the problem as follows....
"I want a program to run in (Windows? Liniux?) which will watch the PC's serial port.
"From time to time, data will appear there. The format will be.... (specify... probably a string of ASCII best)
"When that happens, I want the ID number extracted from it. It will be in the following form... (specify... this time string-of-ASCII needed. Preferably something with only digits or letters, and no more than a stated length long)
"Once the app has that ID number, I want it to look for a JPEG that name. (E.g.: If the serial port data yielded an ID of 7672191, then look for a file called "7672191.jpg"), and put that on the screen."
So! There's all you need! I hope some nice person out there will add a reply to that post saying "I can write that for you!"......