image_single has been deprecated in favour of image_speed and image_index since GM6, well over 7 years ago. It is only there for backwards compatibility (for GM5), not for mainstream use. That's what deprecated means, and that's why it's not in the manual.
Why is image_single not in the manual and why should image_speed and image_index be used instead? I'm using image_single in Studio and I have no problems.