Ok, on the line:
If y < tiw Then y = tiwchange it to:
If y < 240 Then y = 240
This will limit player y position below of aliens positions.
Also you may want to limit power y position, just change this line:
image_y=Rnd(0,585+ImageHeight(img_power))to
image_y=Rnd(240,585+ImageHeight(img_power))