Um, it should work fine using
PHP Code:
SCRIPT
function onPlayerEnters() {
sleep(3);
findimg(200).image = "newimage.png";
}
SCRIPTEND
Very rarely I come across a situation where I cannot access sprite indexes using findimg(200), but you might need to "enable drawing" on the sprite index by adding something like:
ROTATIONEFFECT 200 1
I've attached a test gani that does what you need, and works fine.