If you want something to stick ot the screen, use changeimgvis with a layer of 4 or higher.
Try using variables such as screenwidth (how wide the graal window is in pixels) and screenheight (how tall the graal window is in pixels) if you want to make it appear 5 pixels from the bottom of the screen or something.
NPC Code:
showimg index,image,screenwidth/2,screenheight/2;
That would make the image appear centered (You can make an algorithim to make the placing more precisely centered on the screen - Thats your job, if thats what your doing.)
If its for a GUI, I suggest using an index of 200 or more.