Something like this:
+ Code SnippetsetVirtualAspect(4.0/3.0)
imageID = loadImage("my_image.png")
do
time = timer()
if time>spawnTime
spawnTime=time+random(1,12)
spr = createSprite(imageID)
setSpritePosition(spr,random(0,100),0)
endif
sync()
loop
You'll need to play about with it to get it how you want it though...