When you name it, try naming without spaces. Make sure you it is in grayscale, because in RGB it will certainly crash. If you have used colours (British spelling :/) such as red or blue in grayscale because they come up gray, with would recommend changing them to the gray equivalent (Doesn't make a lot of sense -.-) <-- Rewording- Change the colours so if it was in RGB it would all be gray.
Is everything on one layer? If not, I would try that as well.