It doesn't mean the image file is missing - it means the alien or laser object is missing, which narrows it down to two possibilites:
1. You haven't created it.
2. You've created it and deleted it somewhere else.
If you're doing any collision checks that would result in either object being deleted, do that last, as once an object has been deleted you can no longer work with it and you'll get the error you described.
If this doesn't help, post some code.
1. You haven't created it.
2. You've created it and deleted it somewhere else.
If you're doing any collision checks that would result in either object being deleted, do that last, as once an object has been deleted you can no longer work with it and you'll get the error you described.
If this doesn't help, post some code.