In general, its all about PRACTICE! What are you doing if it's only 15 minutes?
MHS/Mac.Rob/Cory/Nossal entrance exam
Nobody can tell you how to write a creative piece, that's sort of the point of the exercise.
+1
A basic analytical approach is as follows (the one I used for yr7 selective/scholarship prep):
Intro (3 lines)
What is the issue?
Where is the problem? How is it a problem?
Why is there a need to fix it (your contention should come in here)? How do you approach the issue (present both sides if it asks you to; don't if it doesn't though)?
Conclusion (2-3 lines)
^Thats fairly basic, but it always gets the job done for these exams

Given you 4 pics, you are to construct an 500 word essay in 15 mins,how? and how to write an argument essay in 15 mins?
somebody plz give me some advice, thx! 
With the pictures, I am pretty sure you don't have to use all of them. Again though, its all about practice and there are infinite ways to write a creative response successfully. My way of practicing was to just take pictures and try and create links between them. Example from the top of my head:


A link between these could be:
A firefighter who is driving in a city (coming back from a job in his firetruck), stops at a STOP sign, sees someone else being mauled by a dog. Uses the fire hydrant (and his own firefighting hose) to get rid of the dog and save the person. The end.
^Okay, thats a pretty lame and basic example skeleton of a story. But in the exam, they are not expecting a Dan Brown novel in 15 mins. Sometimes its good to keep things simple. After
a while practice, you'll be able to string a basic story together from any 3 or 4 pics.
Worst ending though, for any essay, is to say "and he woke up, and it was all just a dream". That's a recipe for failure...