I don't think that's entirely true. I've been looking through the source of RSBot, and there was a method to capture a screenshot, so I know it's not impossible. I don't know if it would be awfully efficient though, when you think about the number of pixels being handled to make the image, then any methods to cycle through all the pixels. I had the idea of images in different areas (MS, MM, INV, Chat), so it would be dealing with far less data. If you'd like to look into how RSBot dealt with it, I've got the source for the screenshot utility
here, you can cycle through the rest of repository to find any other bits that you might find useful.