ActionScript 2.0 :: Replacing A Picture By A Default One If Missing

In this script, each time the user press the space bar, a new picture is displayed
(in my real application, the picture is taken from a database. Here for this example a static picture is OK) If the picture doesn't exist, a default picture is loaded.

Here is the corresponding code, each time you press the space bar, the "test.png" picture is loaded If it isn't present, it loads ""noscreen.png""

I thought that it was working correctly (even if I had a doubt on a potential loop problem in the function, since a new picture load was called when I'was already in the loader) Anyway, this seemed to work...until I discovered that if the "noscreen.png" was missing too...the CPU began to fly off!

You can do the test by yourself, watching the task manager, press on the space bar continually

1) with the "noscreen.png" present...no problems

2) with the "noscreen.png" missing, the CPU flies..and never get down!

I suppose that this is because it looks after a "test.png" picture, then has the instruction to load a "noscreen.png" if it's missing ...but this last file is missing itself...so it's looping continually...isn't it?

How would you do to load a picture, and subsitute it by another one if missing
...with the fact that this last one could miss from the user PC (oh, yes...it will be a projector app) too and it that case it must be skipped simply

Code:
MC_loader = new MovieClipLoader();
preload = new Object();
MC_loader.addListener(preload);

[Code].....